Enhanced system and method for string balancing

ABSTRACT

A photovoltaic system, including: a plurality of photovoltaic panels having outputs connected in series as a string to provide a string output; a converter coupled to the string to receive the string output as an input and generate a direct current output from the input; a series connection of the string output and the direct current output; and a bus powered at least in part by the series connection of the string output and the direct current output.

RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 16/881,540 filed May 22, 2020, which is acontinuation application of U.S. patent application Ser. No. 15/172,996,filed Jun. 3, 2016, issued as U.S. Pat. No. 10,673,244 on Jun. 2, 2020,and entitled “Enhanced System and Method for String-Balancing”, which isa continuation application of U.S. patent application Ser. No.13/418,279, filed Mar. 12, 2012, issued as U.S. Pat. No. 9,368,965 onJun. 14, 2016 and entitled “Enhanced System and Method forString-Balancing”, which claims the benefit of the filing dates of Prov.U.S. Pat. App. Ser. No. 61/512,785, filed Jul. 28, 2011 and entitled,“System and Method for Reducing the Number and Cost of Local ManagementUnits in a Distributed Power Generation Site,” and Prov. U.S. Pat. App.Ser. No. 61/547,655, filed Oct. 14, 2011 and entitled “Enhanced Systemand Method for String Balancing”, the entire disclosures of which areeach incorporated herein by reference.

FIELD OF THE TECHNOLOGY

At least some embodiments of this disclosure relate to photovoltaicsystems in general, and more particularly but not limited to, improvingthe energy production performance of photovoltaic systems.

BACKGROUND

A traditional maximum power point tracking (MPPT) algorithm sees a solararray as if it were a single solar module. MPPT can pull and pushcurrent on all strings and solar modules in a solar array in anequivalent fashion. As such, if solar modules in the solar array operateat different working points on the I-V curve, due to differences ininstallation, fabrication, or degradation over time, an MPPT algorithmmay not be able to find the maximum power point (MPP) for the solararray.

U.S. Pat. No. 7,602,080, issued on Oct. 13, 2009 and entitled “Systemsand Methods to Balance Solar Panels in a Multi-Panel System,” disclosesa local management unit for a solar module. The local management unithas a controller for controlling the operation of the solar module and alink module unit to provide connectivity to a power bus for energydelivery and/or for data communications. The entire disclosure of U.S.Pat. No. 7,602,080 is incorporated herein by reference.

SUMMARY OF THE DESCRIPTION

Systems and methods to control power generators individually withreduced cost are described herein. Some embodiments are summarized inthis section.

In one of many embodiments disclosed in the disclosure, an apparatusincludes a plurality of direct current converters with each converterconnected to a respective solar panel of a plurality of solar panels andconfigured to receive direct current power from the panel connectedthereto. The direct current outputs of the direct current converters areconnected in series to provide power generated by the solar panels.

In one embodiment, the apparatus includes a control circuitry includinga communications unit to receive control signals from a location remotefrom the control circuit, and a controller coupled with thecommunications unit to control the direct current converters accordingto the control signals. The control signals can include duty cyclesignals used to operate each of the respective converters connected tothe controller. In one embodiment, the apparatus can further includesensors configured to measure operating states of the solar panels; andthe communications unit is configured to transmit measurements generatedby the sensors to a remote management unit from which the controlsignals are received.

In one of many embodiments disclosed in the disclosure, the apparatus isconfigured to be mounted on one solar panel of the plurality of solarpanels connected thereto, and remaining solar panels of the plurality ofsolar panels are connected to the apparatus via wires that provide inputto respective direct current converters. In one embodiment, the solarpanel to which the apparatus is secured is the panel physically locatedin the center position in the layout of the plurality of solar panelsconnected to the apparatus.

In one embodiment, an apparatus includes a plurality of direct currentconverters, with each converter of the direct current converters havingan input configured to be connected to a separate photovoltaic panel ofa plurality of photovoltaic panels, and a direct current output toprovide electrical power generated by the separate photovoltaic panel.Outputs of the direct current converters are connected in series in theapparatus, and a single communications unit is coupled between thedirect current converters and a system management unit. The systemmanagement unit is configured to control operations of the directcurrent converters via communicating with the communications unit.

In one embodiment, a plurality of controllers are coupled with theplurality of direct current converters respectively to control dutycycles of the direct current converters.

In one embodiment of the disclosure, systems include a plurality ofsolar panels, and a combined local management unit (CLMU) connectedbetween the plurality of solar panels and a first output that isconfigured to provide electrical power generated by the plurality ofsolar panels. The CLMU includes a plurality of converters each connectedto a respective solar panel of the plurality of solar panels to receivedirect current input and to provide a second direct current output. Thesecond direct current outputs from the converters are connected inseries to provide the first output.

In one embodiment, the CLMU further includes a communications unitconfigured to communicate with a remote management unit separate fromthe CLMU. A controller of the CLMU is coupled with the communicationsunit to control operations of the converters in accordance withcommunications with the remote management unit.

In one embodiment, the CLMU is secured to a single solar panel of theplurality of solar panels. The CLMU can be secured to the solar panelphysically located at a middle position of the plurality of solarpanels. In one embodiment, the CLMU can include a current sensor, aplurality of voltage sensors, and a remote management unit. The currentsensor can be configured to measure a direct current provided by thefirst output. The plurality of voltage sensors can be configured tomeasure voltages provided by the plurality of solar panels. The remotemanagement unit can be configured to communicate with the communicationsunit of the CLMU to control operations of the plurality of convertersfor the plurality of respective solar panels. In one embodiment, theremote management unit can be further configured to receive measurementsindicating operating states of the solar panels, determine duty cyclesof the converters, and communicate data identifying the duty cycles tothe CLMU's communications unit. The controller can be configured tooperator the converters in accordance with the data identifying the dutycycles.

In one embodiment, the communications unit is configured to communicatewith the remote management unit via a wired connection for providing thefirst output. Alternatively or in combination, the communications unitis configured to communicate with the remote management unit through awireless connection.

In one embodiment, the remote management unit can be configured toinstruct the controller, via the communications unit, to operate theconverters in states to maximize power generated by the plurality ofsolar panels.

In one embodiment, the CLMU connects the plurality of solar panels toform a first string of solar panels. The system can have a plurality ofstrings, including the first string. The embodiment can further includea parallel direct current bus configured to receive outputs from aplurality of strings in parallel.

In one embodiment, the system includes a combiner box housing theparallel direct current bus. The combiner box can include one or morereceptacles configured to receive a removable modular unit having aplurality of first terminals and a second terminal. When a first stringis connected to a receptacle in the combiner box, the removable modularunit is configured to receive first power from the first string via theplurality of first terminals, and feed the first power to the paralleldirect current bus through the second terminal. In one embodiment, theremovable modular unit is further configured to measure power inputreceived at the respective first terminals, selectively disconnect thefirst string from the parallel direct current bus based oncommunications with the remote management unit, or convert a voltage ofthe first string for outputting to the parallel direct current bus.

In one embodiment, a method for string balancing a plurality ofphotovoltaic panels connected in a plurality of strings in turnconnected in parallel to a bus include sensing a string voltage on eachof the plurality of strings, setting the string having a highest voltageas V_(Smax); and up-converting voltage on the remaining strings to matchthe highest string voltage, V_(Smax). In such an embodiment, theup-converting includes providing a dc-to-dc converter in series witheach string to match voltage to V_(Smax).

In one embodiment, the up-converting includes connecting a V_(supply) toeach dc-to-dc converter to provide additional voltage to raise or matchthe bus voltage at V_(Smax). The up-converting may include providing aportion of string output as the V_(supply) to the dc-to-dc converter.Alternatively, in one embodiment the V_(supply) is a separate voltagesource.

In one embodiment a photovoltaic distribution system may includeplurality of photovoltaic panels connected in a plurality of strings inturn connected in parallel to a load bus. Each string has a dc-to-dcconverter connected in series to the load bus, wherein each converter isconfigured to match string current and voltage to the one string havinga highest voltage. Thus the bus voltage in this embodiment would be thatmaximum string voltage, designated V_(Smax). In one embodiment thesystem includes one or more of the dc-to-dc converters having a separatevoltage supply connected to supplement string voltage to achieveV_(Smax).

The disclosure includes methods and apparatuses which perform thesemethods, including data processing systems which perform these methods,and computer readable media containing instructions which when executedon data processing systems cause the systems to perform these methods.

Other features will be apparent from the accompanying drawings and fromthe detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not limitation inthe figures of the accompanying drawings in which like referencesindicate similar elements.

FIGS. 1-3B illustrate local management units according to someembodiments.

FIG. 4A illustrates a photovoltaic system according to one embodiment.

FIG. 4B illustrates an embodiment of a solar array along with aninverter and a string combiner.

FIG. 5 illustrates a solar panel according to one embodiment.

FIGS. 6-8 show methods to improve performance of a photovoltaic systemaccording to some embodiments.

FIG. 9 illustrates a local management unit according to one embodiment.

FIG. 10A is a plot of carrier frequency for a local management unitaccording to one embodiment.

FIG. 10B illustrates a subsystem including a local management unitaccording to one embodiment.

FIG. 11A illustrates a photovoltaic system according to one embodiment.

FIG. 11B illustrates a receiver of a photovoltaic system according toone embodiment.

FIG. 12 illustrates a local management unit according to one embodiment.

FIGS. 13-18 illustrate operation of the local management unitillustrated in FIG. 12 .

FIG. 19 illustrates a local management unit and transmission modulatoraccording to one embodiment.

FIG. 20 illustrates an exemplary inverter current controlled by amaximum power point tracking algorithm.

FIG. 21 illustrates exemplary solar module voltages for strong and weaksolar modules.

FIG. 22 illustrates an exemplary composite I-V curve for solar modulesin a solar array.

FIG. 23 illustrates exemplary plots of current changes seen on twostring buses when connected in parallel.

FIG. 24 illustrates an exemplary current versus time diagram for astronger and weaker string bus when the voltage to the string busesfluctuates.

FIG. 25 illustrates an exemplary composite I-V curve for string buses ina solar array.

FIG. 26 illustrates an embodiment of a method of maximizing the poweroutput of a solar array by (1) balancing current outputs of solarmodules, (2) balancing voltage outputs of string buses, and (3) applyingan MPPT algorithm to the solar array.

FIG. 27 illustrates an I-V curve for a string bus where all solarmodules (2702) are operating at their ideal outputs.

FIG. 28 illustrates an I-V curve for a string bus where two solarmodules are operating as weak solar modules.

FIG. 29 illustrates an I-V curve for a string bus implementing thesystems and methods of this disclosure.

FIG. 30 illustrates an I-V curve for a solar array where all stringbuses (3002) are operating at their ideal outputs.

FIG. 31 illustrates a composite I-V curve for string buses in a solararray where two string buses are operating as weak string buses.

FIG. 32 illustrates an I-V curve for a solar array implementing thesystems and methods of this disclosure.

FIG. 33 illustrates an I-V curve for a solar array having string busesbalanced via increasing string bus output voltage.

FIG. 34 illustrates an embodiment of a solar array according to thepresent disclosure.

FIG. 35 illustrates an embodiment of a local string management unit(LSMU).

FIG. 36 illustrates another embodiment of an LSMU.

FIG. 37 illustrates an embodiment of a method for balancing strings in asolar array using LSMUs.

FIG. 38 illustrates an embodiment of a modular unit system used toimprove the energy production performance of photovoltaic systems.

FIGS. 39A and 39B illustrate two embodiments of a modular unit'scircuitry that can be used in the modular unit system of FIG. 38 .

FIG. 40 illustrates an overview of an embodiment of a modular unitsystem used to improve the energy production performance of photovoltaicsystems having a housing to encase the modular unit system.

FIG. 41 illustrates an embodiment of a method for improving the energyproduction performance of photovoltaic systems.

FIG. 42 illustrates an overview of an exemplary modular unit in form ofa blade for use in a modular unit system used to improve the energyproduction of photovoltaic systems.

FIG. 43 illustrates an alternative embodiment of a modular unit systemused to improve the energy production performance of photovoltaicsystems.

FIG. 44 illustrates an embodiment of a method for utilizing a modularunit system in improving the energy production performance ofphotovoltaic systems.

FIG. 45 illustrates an embodiment of a system utilizing combined localmanagement units (CLMUs) to improve the energy production performance ofphotovoltaic systems.

FIG. 46 illustrates an exemplary embodiment of a system having threesolar panels connected to a solar string through a single CLMU.

FIG. 47 illustrates an exemplary embodiment of a CLMU used to improvethe energy production performance of a group of solar panels connectedthereto.

FIG. 48 illustrates an exemplary system for string balancing of aphotovoltaic panel system in accordance with the present disclosure.

FIGS. 49A and 48B illustrate an exemplary bus architecture for stringbalancing with an up-converter and including loss calculations.

FIGS. 50A and 50B illustrate another exemplary bus architecture forstring balancing and including exemplary loss calculations in accordancewith the present disclosure.

FIGS. 51A and 51B illustrates another exemplary bus architecture forstring balancing with exemplary loss calculations.

DETAILED DESCRIPTION

The following description and drawings are illustrative and are not tobe construed as limiting. Numerous specific details are described toprovide a thorough understanding. However, in certain instances, wellknown or conventional details are not described in order to avoidobscuring the description. References to one or an embodiment in thepresent disclosure are not necessarily references to the sameembodiment; and, such references mean at least one.

Reference in this specification to “one embodiment” or “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiment is included in at least one embodimentof the disclosure. The appearances of the phrase “in one embodiment” invarious places in the specification are not necessarily all referring tothe same embodiment, nor are separate or alternative embodimentsmutually exclusive of other embodiments. Moreover, various features aredescribed which may be exhibited by some embodiments and not by others.Similarly, various requirements are described which may be requirementsfor some embodiments but not other embodiments. As a result, thisspecification represents a disclosure of all possible combinations offeatures described herein, except that certain combinations are excludedby reasons of mutually exclusive relationships in features, where themutual exclusiveness is either explicitly identified in thisspecification or is apparent from the description of the respectivefeatures.

When solar modules are connected in series or mesh configuration, therecan be a problem in which weaker modules not only produce less energybut also affect other modules in the same string or wiring section. Bymeasuring one can determine that a few modules are weaker than theothers in most commercially installed strings. Thus, the string isgenerating less power than the sum available at each module if moduleswere operated separately.

At least one embodiment of the present disclosure provides methods andsystems to switch on and off weak modules in the string in a way thatthe current on the string bus from the good modules won't be affected bythe weak modules.

The present invention allows transmission of data from solar modules toa central (or system controller management) unit and other localmanagement units in an energy production or photovoltaic system withoutadding significant cost. One embodiment of the present inventioninvolves using the typically undesired electrical noise produced whenoperating local management units (sometimes referred to as “controllers”or “converters”) to act as a carrier system for data to be transferred.As there are a multitude of solar modules, each can be run on a slightlydifferent frequency. Such an approach allows a receiver in the energyproduction or photovoltaic system to identify the carrier signal of eachlocal management unit separately. This approach has the added benefit ofreducing the overall system noise, because each local management unitsends “noise energy” in a different part of the spectrum, thus helpingto avoid peaks.

FIGS. 1-3B illustrate local management units according to someembodiments. In FIGS. 1-3B, local management units (101) are used toswitch on and off the solar module (102) periodically to improve theenergy production performance of the photovoltaic systems connected, atleast in part, in series. A local management unit can be variouslyreferred to as a solar module controller (or converter) or link moduleunit. One example of a local management unit is any of the various localmanagement units (solar module controllers) offered by Tigo Energy, Inc.of Los Gatos, Calif.

In FIG. 1 , a management unit (101) is local to the solar module (102)and can be used to periodically couple the solar module (102) to theserial power bus (103) via the switch Q1 (106), to improve the totalpower output for the string of solar modules connected to the serialpower bus in series.

The local management unit (LMU) (101) can include a solar modulecontroller to control the operation of the solar module (102) and/or alink module unit to provide connectivity to the serial power bus (103)for energy delivery and/or for data communications.

In one embodiment, the command to control the operation of the switch Q1(106) is sent to the local management unit (101) over the photovoltaic(PV) string bus (power line) (103). Alternatively, separate networkconnections can be used to transmit the data and/or commands to/from thelocal management unit (101).

In FIGS. 1 and 2 , the inputs (104 a, 104 b, 104 c) to the localmanagement unit (101) are illustrated separately. However, the inputs(104 a, 104 b, 104 c) are not necessarily communicated to localmanagement unit (101) via separate connections. In one embodiment, theinputs are received in the local management unit via the serial powerbus (103).

In FIG. 1 , the solar module (102) is connected in parallel to thecapacitor C1 (105) of the local management unit (101). The diode D1(107) of the local management unit (101) is connected in series in theserial power bus (103) which can or may not be part of an overall meshconfiguration of solar modules. The switch Q1 (106) of the localmanagement unit can selectively connect or disconnect the solar module(102) and the capacitor C1 (105) from a parallel connection with thediode D1 (107) and thus connect or disconnect the solar module (102)from the serial power bus (103).

In FIG. 1 , a controller (109) of the local management unit (101)controls the operation of the switch (106) according to the parameters,such as duty cycle (104 a), phase (104 b) and synchronization pulse (104c).

In one embodiment, the controller (109) receives the parameters (104 a,104 b, 104 c) from a remote management unit via the serial power bus(103) or a separate data communication connection (e.g., a separate databus or a wireless connection). In some embodiment, the controller (109)can communicate with other local management units connected on theserial power bus (103) to obtain operating parameters of the solarmodules attached to the serial power bus (103) and thus compute theparameters (e.g., 104 a and 104 b) based on the received operatingparameters. In some embodiment, the controller (109) can determine theparameter (e.g., 104 a and 104 b) based on the operating parameters ofthe solar module (102) and/or measurements obtained by the controller(109), without communicating with other local management units of othersolar modules, or a remote system management unit.

In FIG. 2 , a system (100) has a local management unit (101) coupled tothe solar module (102). The local management unit (101) is connectedbetween the solar module (102) and the string bus (103) to improve thetotal power output for the whole string on the serial power bus (103).Commands to the local management unit (101) can be sent over thephotovoltaic (PV) string bus (power line) (103). To make the figure moreclear, the inputs (104 a, 104 b, 104 c) to the controller (109) of thelocal management unit (101) were drawn separately, which does notnecessarily indicate that the inputs (104 a, 104 b, 104 c) are providedvia separate connections and/or from outside the local management unit(101). For example, in some embodiments, the controller (109) cancompute the parameters (104 a, 104 b, 104 c) based on measurementsobtained at the local management unit (101), with or without datacommunications over the serial power bus (103) (or a separate datacommunication connection with other management units).

In FIG. 2 , the local management unit (101) is connected in one side tothe solar module (102) in parallel and on the other side in series to astring of other modules, which can or may not be part of an overall meshconfiguration. The local management unit (101) can receive, amongothers, three inputs or types of input data, including a) requested dutycycle (104 a), which can be expressed as a percentage (e.g., from 0 to100%) of time the solar module (102) is to be connected to the serialpower bus (103) via the switch Q1 (106), b) a phase shift (104 b) indegrees (e.g., from 0 degree to 180 degree) and c) a timing orsynchronization pulse (104 c). These inputs (e.g., 104 a, 104 b and 104c) can be supplied as discrete signals, or can be supplied as data on anetwork, or composite signals sent through the power lines orwirelessly, and in yet other cases, as a combination of any of theseinput types.

In FIG. 2 , the local management unit (101) periodically connects anddisconnects the solar module (102) to and from the string that forms theserial power bus (103). The duty cycle (104 a) and the phase (104 b) ofthe operation of the switch Q1 (106) can be computed in a number of waysto improve the performance of the system, which will be discussedfurther below.

In FIG. 2 , the local management unit (101) includes a capacitor C1(105) and a switch Q1 (106), as well as a diode D1 (107). In FIG. 2 ,the diode D1 (107) is supplemented with an additional switch Q2 (108),which acts as a synchronous rectifier to increase efficiency. In oneembodiment, the additional switch Q2 (108) is open (turned off) when theswitch Q1 (106) is closed (turned on) to attach the solar module (102)(and the capacitor C1 (105)) to the serial power bus (103).

In some cases, a filter (not shown), including a serial coil and aparallel capacitor, is also used. The filter can be placed at the localmanagement unit or placed just before the fuse box or inverter, or bepart of either one of those.

In FIG. 2 , the controller (109) is used to process the input signals(e.g., 104 a, 104 b, 104 c) and drive the switches Q1 (106) and Q2(108). In one embodiment, the controller (109) is a small single chipmicro controller (SCMC). For example, the controller (109) can beimplemented using Application-Specific Integrated Circuit (ASIC) orField-Programmable Gate Array (FPGA). The controller (109) can even beimplemented in discrete, functionally equivalent circuitry, or in othercases a combination of SCMC and discrete circuitry. It will be generallyreferred to as single chip micro controller (SCMC) herein, but anyimplementation can be used.

In one embodiment, the controller (109) is coupled to the solar module(102) in parallel to obtain power for processing; and the controller(109) is coupled to the serial power bus (103) to obtain signalstransmitted from other management units coupled to the serial power bus(103).

By switching the module (102) (or groups of cells, or a cell) on and offto the string periodically, the local management unit (101) can lowerthe voltage reflected to the string bus (103) (e.g., a lower averagevoltage contributed to the string bus) and can cause the currentreflected to the string bus (103) to be higher, nearer the level itwould be if the module was not weak, generating a higher total poweroutput.

In one embodiment, it is preferable to use different phases to operatethe switches in different local management units on a string to minimizevoltage variance on the string.

In FIG. 3A, the local management unit (101) provides two connectors (112and 114) for serial connections with other local management unit (101)to form a serial power bus (103). The controller (109) controls thestates of the switches Q1 (106) and Q2 (108).

In FIG. 3A, when the controller (109) turns on the switch (106), thepanel voltage and the capacitor C1 (105) are connected in parallel tothe connectors (112 and 114). The output voltage between the connectors(112 and 114) is substantially the same as the output panel voltage.

In FIG. 3A, during the period the switch (106) is turned off (open), thecontroller (109) turns on (closes) the switch (108) to provide a patharound the diode D1 (107) to improve efficiency.

In FIG. 3A, when the switch (106) is turned off (open), the panelvoltage charges the capacitor C1 (105), such that when the switch (106)is turned on, both the solar panel and the capacitor (105) providescurrents going through the connectors (112 and 114), allowing a currentlarger than the current of the solar panel to flow in the string (theserial power bus (103)). When the switch (106) is turned off (open), thediode D1 (107) also provides a path between the connectors (112 and 114)to sustain the current in the string, even if the switch (108) is offfor some reasons.

In one embodiment, the controller (109) is connected (not shown in FIG.3A) to the panel voltage to obtain the power for controlling theswitches Q1 (106) and Q2 (108). In one embodiment, the controller (109)is further connected (not shown in FIG. 3A) to at least one of theconnectors to transmit and/or receive information from the string. Inone embodiment, the controller (109) includes sensors (not shown in FIG.3A) to measure operating parameters of the solar panel, such as panelvoltage, panel current, temperature, light intensity, etc.

FIG. 3B shows an alternative three terminal implementation of the localmanagement unit 101 shown in FIG. 3A. In FIG. 3B, a panel voltage (180)is connected to terminals (182, 184). Terminals (182, 186) are connectedto the string bus (103). A module driver (110) and a single chip microcontroller (SCMC) control the switches Q1 and Q2. Under normal operatingconditions, Q1 is on to allow normal operation of the system. Whenstring current exceeds source capability, and as a result source voltagedrops, Q1 and Q2 start a PWM (pulse width modulation) operation undercontrol of the module driver (110). PWM involves modulation of dutycycle to control the amount of power sent to the load. This allowsstring current to remain constant, and input voltages can be maintainedat the maximum power point. This implementation protects transistorsduring low voltage or short situations. In one embodiment, a single chipmicro controller (SCMC) (109) can be connected in parallel to the diodeD1 (107) to function in the manner of the SCMC 109 as described above.In one embodiment, the module driver (110) and the single chip microcontroller (SCMC) (109) can be integrated in a single controller asshown in, for example, FIG. 3A. As discussed above, single chip microcontroller (SCMC) (109) can receive the inputs (104 a, 104 b, 104 c). Asshown in FIG. 3B, in one embodiment, the inputs (104 a, 104 b, 104 c)are provided with a communications interface (not shown) coupled to amaster controller (not shown). In one embodiment, other inputs (104 n)constituting information about other operating parameters can also becommunicated to the single chip micro controller (SCMC) (109) from thecommunications interface. In one embodiment, the other inputs (104 n)can be information that is communicated bi-directionally. As discussedabove, the power supply connections in the figures, including FIG. 3B,are not necessarily shown for purposes of clarity.

FIGS. 4A and 4B illustrate a photovoltaic system (200) according to oneembodiment. In FIGS. 4A and 4B, the photovoltaic system 200 is builtfrom a few components, including photovoltaic modules (201 a, 201 b, . .. , 201 n), local management unit units (202 a, 202 b, . . . , 202 n),an inverter (203), and a system management unit (204).

In one embodiment, the system management unit (204) is part of theinverter (203), the combiner box (206), a local management unit, or astand-alone unit. The solar modules (201 a, 201 b, . . . , 201 n) areconnected in parallel to the local management units (202 a, 202 b, . . ., 202 n) respectively, which are connected in series to form a stringbus (205), which eventually is connected to an inverter (203) and themanagement unit (204). The solar module (201 a), for example, isconnected to the local management unit (202 a) by the terminals (182,184, 186) (FIG. 3B). As shown in FIGS. 4A and 4B, in one embodiment, theterminal (182), which connects to the panel voltage and the stringvoltage, is connected to the depicted left connection between the solarmodule (201 a) and the local management unit (202 a) and connected tothe depicted left connection between the local management unit (202 a)and the string bus (205). The terminal (184), which is connected to thepanel voltage, is connected to the depicted right connection between thebetween the solar module (201 a) and the local management unit (202 a).The terminal (186), which is connected to the string voltage, isconnected to the depicted right connection between the local managementunit (202 a) and the string bus (205).

In FIGS. 4A and 4B, the string bus (205) can be connected to theinverter (203) directly or as part of a mesh network or combiner boxesor fuse boxes (not shown). An isolated local management unit can be usedas a combiner box (206) to adjust all voltages before connecting to theinverter (206); or, a single or multi-string inverter can be used. Tolimit the changes in the voltage of the bus, the management unit (204)can assign a different phase for each of the local management units (202a, 202 b, . . . , 202 n). In one embodiment, at any given time, amaximum of a predetermined number of solar modules (e.g., one singlesolar module) are disconnected from the string bus (205).

In one embodiment, beyond the module connection the local managementunits can have the signal inputs, including but not limited to dutycycle (104 a), phase (104 b) and synchronization pulse (104 c) (e.g., tokeep the local management units synchronized). In one embodiment, thephase (104 b) and the synchronization pulse (104 c) are used to furtherimprove performance, but the local management unit (101) can workwithout them.

In one embodiment, the local management unit can provide output signals.For example, the local management unit (101) can measure current andvoltage at the module side and optionally measure current and voltage inthe string side. The local management unit (101) can provide othersuitable signals, including but not limited to measurements of light,temperature (both ambient and module), etc.

In one embodiment, the output signals from the local management unit(101) are transmitted over the power line (e.g., via power linecommunication (PLC)), or transmitted wirelessly.

In one embodiment, the system management unit (204) receives sensorinputs from light sensor(s), temperature sensor(s), one or more each forambient, solar module or both, to control the photovoltaic system (200).In one embodiment, the signals can also include synchronization signals.For example, a management unit can send synchronization signalsperiodically to set the timing values, etc.

Using the described methods the local management unit can be a verynon-expensive and reliable device that can easily increase thethroughput of a photovoltaic solar system by a few (e.g., signal or lowdouble digits) percentage points. These varied controls also allowinstallers using this kind of system to control the VOC (open circuitvoltage) by, for example by shutting off some or all modules. Forexample, by using the local management units of the system, a fewmodules can be disconnected from a string if a string is getting to theregulatory voltage limit, thus more modules can be installed in astring.

In some embodiments, local management units can also be used within thesolar panel to control the connection of solar cells attached to stringsof cells within the solar panel.

FIG. 5 illustrates a solar panel according to one embodiment. In oneembodiment, the solar panel (300) has a few strings of solar cells(e.g., three solar cell strings per module). In FIG. 5 , a localmanagement unit (101) can be applied to a group of cells (301) within astring of an individual solar panel (300), or in some cases to each cell(301) in a solar panel (300).

In FIG. 5 , a group of solar cells (301) that are attached to a localmanagement unit (101) can be connected to each other in series, inparallel, or in a mesh configure. A number of local management units(101) connect the groups of the solar cells (301) in a string to provideoutput for the solar panel (300).

Some embodiments of the disclosure include methods to determine the dutycycles and/or phases for local management units connected to a string ormesh of solar modules.

In some embodiments, the duty cycle of all local management units in astring or mesh can be changed, to increase or decrease the stringvoltage. The duty cycles can be adjusted to avoid exceeding the maximumvoltage allowed. For example, the maximum voltage can be limited by thecombiner box (206), the inverter (203), or any other load connected tothe string bus (205), or limited by any regulations applicable to thatsystem. In some embodiments, the duty cycles are adjusted to align thevoltage of multiple strings.

In some embodiments, the duty cycle of one local management unit (101)in a string can be changed to cause higher current in that localmanagement unit (101) and overall higher power harvesting.

In one embodiment, the duty cycles are computed for the solar modulesthat are connected to a string via the corresponding local managementunits. The duty cycles can be calculated based on the measured currentand voltages of the solar modules and/or the temperatures.

After an initial set of duty cycles is applied to the solar modules, theduty cycles can be further fine tuned and/or re-adjusted to changes,such as shifting shading etc., one step a time, to improve powerperformance (e.g., to increase power output, to increase voltage, toincrease current, etc.). In one embodiment, target voltages are computedfor the solar modules, and the duty cycles are adjusted to drive themodule voltage towards the target voltages.

The methods to compute the duty cycles of the solar modules can also beused to compute the duty cycles of the groups of solar cells within asolar module.

FIGS. 6-8 show methods to improve performance of a photovoltaic systemaccording to some embodiments.

In FIG. 6 , at least one operating parameter of a solar energyproduction unit coupled to a string via a management unit is received(401) and used to identify (403) a duty cycle for the management unit toconnect the solar energy production unit to string. The solar energyproduction unit can be a solar module, a group of solar cells within asolar module, or a single solar cell in a string in a solar module. Theduty cycle is adjusted (405) to optimize the performance of the solarenergy production unit and/or the string.

For example, the duty cycle can be adjusted to increase the current inthe string and/or the solar energy production unit, to increase theoutput power of the string and/or the solar energy production unit, toincrease the voltage of the solar energy production unit, etc.

In FIG. 7 , the operating voltages of a plurality of solar panelsconnected in series are received (421) and used to identify (423) asecond solar panel having the highest operating voltage (highest outputpower) in the string.

In FIG. 7 , a duty cycle of a first solar panel is computed (425) basedon a ratio in operating voltage between the first and second solarpanels. Alternatively, the duty cycle can be computed based on a ratioin output power between the first and second solar panels.Alternatively, the duty cycle can be computed based on a ratio betweenthe first and second solar panels in estimated/computed maximum powerpoint voltage. Alternatively, the duty cycle can be computed based on aratio between the first and second solar panels in estimated/computedmaximum power point power.

The duty cycle of the first solar panel is adjusted (427) to improve theperformance of the first solar energy production unit and/or the string,until a decrease in the operating voltage of the second solar panel isdetected. For example, the duty cycle of the first solar panel can beadjusted to increase the total output power of the string, to increasethe current of the string, to increase the current of the first solarpanel, to drive the voltage of the first solar panel towards a targetvoltage, such as its maximum power point voltage estimated based on itscurrent operating parameters, such as temperature or a voltagecalculated using its estimated maximum power point voltage.

In FIG. 7 , in response to the detected decrease in the operatingvoltage of the second solar panel which had the highest operatingvoltage, the adjustment in the duty cycle of the first solar panel thatcauses the decrease is undone/reversed (429).

In FIG. 7 , the duty cycle of the second solar panel is optionallydecreased (431) to increase the operating voltage of the second solarpanel. In some embodiments, the strongest solar panel (or strong panelswithin a threshold from the strongest panel) is not switched off line(e.g., to have a predetermined duty cycle of 100%).

In one embodiment, the duty cycle of the second solar panel isrepeatedly decreased (429) until it is determined (431) that thedecrease (429) in the duty cycle of the second solar panel cannotincrease the voltage of the second solar panel.

In FIG. 8 , operating parameters of a plurality of solar panelsconnected in a string are received (441) and used to identify (443) afirst maximum power point voltage of a first solar panel. A second solarpanel having the highest operating voltage (or output power) in thestring is identified. A second maximum power point voltage of the secondsolar panel is identified (447) based on the received operatingparameters and used to compute (449) a target voltage for the firstsolar energy production unit. In one embodiment, the target voltage is afunction of the first and second maximum power point voltages and thehighest operating voltage identified (445) in the second solar panel inthe string. The duty cycle of the first solar energy production unit isadjusted to drive the operating voltage of the first solar panel towardsthe target voltage.

Alternatively, the target voltage can be set as the first maximum powerpoint voltage of the first solar panel.

In one embodiment, to adjust voltage a same factor is applied to allmodules in that string. For example, in a case of a first module A1 thatis producing only 80%, and the voltage of the whole string needs to be5% lower, the duty cycle of A1 is 80% multiplied the duty cycle appliedto the whole string (which is Y in this example) so module A1 then hasY×0.8 as duty cycle.

In some embodiments, the system management unit (204) and/or the localmanagement units (e.g., 202 a, 202 b, . . . , 202 n) are used solely orin combination to determine the parameters to control the operations ofthe switches.

For example, in one embodiment, a system management unit (204) is the“brain” of the system, which decides on the duty cycle and phaseparameters.

For example, in another embodiment, each local management unitbroadcasts information to the other local management units on the stringto allow the individual local management units to decide their own dutycycle and phase parameters.

In some embodiment, a local management unit can instruct one or moreother local management units to adjust duty cycle and phase parameters.For example, the local management units on a string bus (205) can electone local management unit to compute the duty cycle and phase parametersfor other local management units on the string.

For example, in some embodiment, the system management unit (204) candetermine one or more global parameters (e.g., a global duty cycle, themaximum power on the string, the maximum voltage on the string, etc.),based on which individual local management units adjust their own dutycycles.

In some embodiments, a local management unit can effectively self manageand determine its own duty cycles without relying upon communicatingwith other management units. For example, the local management unit canadjust its duty cycle for connecting its solar module to the string tooperate the solar module at the maximum power point. No local managementunit is in control over the system, and each adjusts its own duty cycle(and thus, its power and voltage.)

In one embodiment, module voltages are measured by the local managementunits in the same string at substantially/approximately the same timeand used to identify the strongest solar module. A strongest solarmodule provides the most power in the string. Since the modules areconnected in series, the solar module having the highest module voltagein the string can be identified as the strongest solar module. In someembodiment, the operating voltage and current of the solar module aremeasured to determine the power of the solar module.

Additional approaches can be implemented to control the voltage, poweroutput, or the efficiency of one or more strings of solar modulecontrollers as described above. In some embodiments, a system controllermanagement unit controls the operation of a plurality of localmanagement units in one or more strings. In some embodiments, one ormore local management units control the operation of a plurality oflocal management units in one or more strings. In some embodiments, thelocal management unit can only control its own operation, or can controlthe operation of itself and other local management units in the samestring.

One or more local management units in a string can have the capabilityto control the operation of other local management units in the samestring. In one embodiment, a single local management unit can beselected to be a controlling local management unit to control aplurality of panels in a string. The controlling local management unitin a string can be selected using any suitable protocol. In oneembodiment, in a string of local management units, the first localmanagement unit that announces its intent to take control of othermodules in the string could become the controlling local managementunit.

In one embodiment, to improve power output by a string, one or morelocal management units can each receive module voltage from all localmanagement units in the same string and identify the strongest localmanagement unit (i.e., the one with the maximum power and voltage). Eachlocal management unit can then set its own duty cycle as a function ofthe received voltage.

In one embodiment, after the highest module voltage V_(m) in the stringis identified, the duty cycle for each module can be computed as afunction of a ratio between the module voltage V of the module and thehighest module voltage V_(m). For example, the duty cycle for a modulecan be computed as 1−((V_(m)−V)/V_(m))=V/V_(m). In one embodiment, aparticular local management unit receives the voltages of all otherlocal management units at the same time or substantially same time(e.g., all voltages are received within an interval of less than onesecond.)

In one embodiment, the system management unit (204) can identify thehighest module voltage from the module voltages received from the localmanagement units (202 a, 202 b, . . . , 202 n), and compute the dutycycles for the corresponding local management units (202 a, 202 b, . . ., 202 n).

In one embodiment, the local management units (202 a, 202 b, . . . , 202n) can report their module voltages on the string bus (205) to allowindividual local management units (202 a, 202 b, . . . , 202 n) toidentify the highest module voltage and compute the duty cycles, withoutrelying upon the system management unit (204).

In one embodiment, one of the local management units (202 a, 202 b, . .. , 202 n) can identify the highest module voltage and/or compute theduty cycles for the other local management units (202 a, 202 b, . . . ,202 n).

In one embodiment, the duty cycles are determined and/or adjustedperiodically (e.g., every 30 seconds). The intervals can take intoaccount various environmental factors (e.g., where shadows on a solarpanel are cast on different parts of the panel over the course of aday).

In one embodiment, after the duty cycles for the solar modules on thestring are set based on the module voltage ratio relative to the highestmodule voltage in the string, the duty cycles can be fine tuned toincrease the power performance. The duty cycles can be fine tuned onestep a time, until a decrease of voltage of the module with the highestpower is detected. In response to the detected decrease, the last changethat caused the decrease can be reversed (undone). The fine tuning ofthe duty cycles can be used to reach the peak performance point (e.g.,for maximum power point tracking).

In one embodiment, after the strongest module is identified, the dutycycles of the solar modules on the string are adjusted until the modulewith the highest power in the string decrease its voltage. Sincedecreasing the duty cycle of a solar module decreases the time periodthe module is connected to the string and thus increases its voltage,the duty cycle of the module with the highest power in the string can bedecreased to increase its voltage, in response to the decrease in itsvoltage caused by the adjustment to the duty cycles of other solarmodules on the string. For example, the duty cycle of the module withthe highest power in the string can be decreased until its voltage ismaximized.

The performance of solar modules can vary significantly withtemperature. A system capable of measuring temperature can implementmethods for controlling the voltage, power output, or the efficiency ofone or more strings of solar module controllers using module temperatureas a factor. In one embodiment, the local management unit measuresmodule and ambient temperatures for some methods to determine the dutycycles. For example, the operating parameters measured at the localmanagement units (e.g., 202 a, 202 b, . . . , 202 n), such as moduletemperature, can be used to compute the estimated voltages of the solarmodules at their maximum power points. For example, a formula presentedby Nalin K. Gautam and N. D. Kaushika in “An efficient algorithm tosimulate the electrical performance of solar photovoltaic arrays,”Energy, Volume 27, Issue 4, April 2002, pages 347-261, can be used tocompute the voltage V_(mp) of a solar module at the maximum power point.Other formulae can also be used. Once the maximum power point voltageV_(mp) of a solar module is computed or estimated, the duty cycle of thesolar module connected to a string can be adjusted to drive the modulevoltage to the computed/estimated maximum power point voltage V_(mp),since decreasing the duty cycle of a solar module normally increases itsvoltage.

In one embodiment, a local management unit can adjust the duty cycle ofthe solar module connected to the local management unit to change themodule voltage to the computed/estimated maximum power point voltageV_(mp), without having to communicate with other management units.

In one embodiment, a local management unit (or a system management unit)can adjust the duty cycle of the solar module connected to the localmanagement unit to perform maximum power point tracking.

In one embodiment, after identifying the strongest module andcomputing/estimating the maximum power point voltage V_(mpm) of thestrongest module, the duty cycle for each module on a string can becomputed as a function of a ratio between the maximum power pointvoltage V_(mp) of the module and the maximum power point voltage V_(mpm)of the strongest module. For example, the duty cycle for a module can becomputed as 1−((V_(mpm)−V_(mp))/V_(mpm))=V_(mp)/V_(mpm). The duty cyclecan be periodically updated, based on the current operating parametersmeasured, and/or fine-tuned until a decrease in the voltage of thestrongest module is detected.

Alternatively, a target voltage for each module on the string can becomputed as a function of a ratio between the maximum power pointvoltage V_(mp) of the module and the maximum power point voltage V_(mpm)of the strongest module. For example, the target voltage for a modulecan be computed as V_(m)×V_(mp)/V_(mpm), where V_(m) is the measuredvoltage of the strongest module. The duty cycle of the module can bechanged to drive the module voltage of the module towards the targetvoltage.

In one embodiment, after identifying the strongest module andcomputing/estimating the maximum power point power P_(mpm) of thestrongest module, the duty cycle for each module on a string can becomputed as a function of a ratio between the maximum power point powerP_(mp) of the module and the maximum power point power P_(mpm) of thestrongest module. For example, the duty cycle for a module can becomputed as 1−((P_(mpm)−P_(mp))/P_(mpm))=P_(mp)/P_(mpm). The duty cyclecan be periodically updated, based on the current operating parametersmeasured, and/or fine-tuned until a decrease in the voltage of thestrongest module is detected, since decreasing the duty cycle normallyincreases the module voltage.

In one embodiment, a target voltage for each module on the string can becomputed as a function of a ratio between the maximum power point powerP_(mp) of the module and the maximum power point power P_(mpm) of thestrongest module. For example, the target voltage for a module can becomputed as V_(m)×P_(mp)/P_(mpm), where V_(m) is the measured voltage ofthe strongest module. The duty cycle of the module can be changed todrive the module voltage of the module towards the target voltage, sincedecreasing the duty cycle normally increases the module voltage.

In one embodiment, the duty cycle for each local management unit ischanged to increase the current of the solar module attached to thelocal management unit (e.g., based on the measurement of the voltage andcurrent of the solar module), until the maximum current is achieved.This method assumes that string maximum power can be achieved with someaccuracy by driving each local management unit to maximum current. Inone embodiment, the voltages and currents of the solar modules aremeasured for tuning the duty cycles for maximum power point tracking forthe string. The measurements of the voltages and currents of the solarmodules also enable the local management units to additionally serve asa module level monitoring system.

The duty cycles can be adjusted by the system management unit (e.g.,204) based on the measurements reported by the local management units(e.g., 202 a, 202 b, . . . , 202 n), or adjusted directly by thecorresponding local management units (e.g., 202 a, 202 b, . . . , 202n).

In one embodiment, during the process of setting and/or tuning the dutycycles, the maximum power point tracking operation by the inverter (203)is frozen (temporarily stopped). Light intensity at the solar modules ismonitored for changes. When the light intensity at the solar modulesstabilizes, the voltage and current of the solar modules are measuredfor the determination of the duty cycles. Then normal operation resumes(e.g., unfreezing of maximum power point tracking operation).

In one embodiment, the local management units measure the voltages andcurrents of the solar modules to determine the power of the solarmodules. After identifying the highest power Pm of the solar module onthe string, the duty cycles of the solar modules on the string aredetermined by the power ratio relative to the highest power P_(m). Forexample, if a module produces 20 percent less power, it will bedisconnected from the string bus about 20 percent of the time. Forexample, if a module produces power P, its duty cycle can be set to1−((P_(m)−P)/P_(m))=P/P_(m).

In one embodiment, a predetermined threshold is used to select the weakmodules to apply duty cycles. For example, in one embodiment, when amodule produces power less than a predetermine percent of highest powerP_(m), a duty cycle is calculated and applied to the solar module. Ifthe module is above the threshold, the module is not disconnected (andthus having a duty cycle of 100%). The threshold can be based on thepower, or based on the module voltage.

In one embodiment, the system management unit (204) finds the dutycycles for the local management units (202 a, 202 b, . . . , 202 n) andtransmits data and/or signals representing the duty cycles to the localmanagement units (202 a, 202 b, . . . , 202 n) via wires or wirelessconnections. Alternatively, the local management units (202 a, 202 b, .. . , 202 n) can communicate with each other to obtain the parameters tocalculate the duty cycles.

In one embodiment, the system management unit (204) knows all thedifferent duty cycles indicated for the local management units (202 a,202 b, . . . , 202 n).

In one embodiment, during power fine tuning, the system management unit(204) sends the appropriate data/signal to the appropriate localmanagement units (202 a, 202 b, . . . , 202 n), and then the systemmanagement unit (204) calculates the total power of the string andcorrects the duty cycle to produce maximum power. Once maximum power isachieved, the duty cycles for the local management units (202 a, 202 b,. . . , 202 n) can be saved in a database and serve as a starting pointfor the corresponding local management units (202 a, 202 b, . . . , 202n) at the same time of day on the next day. Alternatively, a localmanagement can store the duty cycle in its memory for the next day.

The stored duty cycles can be used when there is a fixed shade on themodules, such as a chimney, a tree, etc., which will be the same shadeon any day at the same time. Alternatively, historical data may not besaved, but can be recalculated from scratch on each run, for exampleevery 30 minutes.

In one embodiment, the light intensity at the solar modules is monitoredfor changes. The duty cycles are calculated when the light intensitydoes not change significantly. If there are changes in sun lightradiation at the solar modules, the system will wait until theenvironment stabilizes before applying or adjusting the duty cycles.

In one embodiment, the system management unit (204) can communicate withthe inverter as well. When the environment is not stable (e.g., when thesun light radiation is changing), the inverter can stop maximum powerpoint tracking. In such a situation, the inverter can be set up for itsload, instead of tracking for maximum power point. Instead of using theinverter to perform maximum power point tracking, the system managementunit (204) and the local management units (202 a, 202 b, . . . , 202 n)are used to set the operating parameters and balance the string.

Alternatively, when the environment is not stable but measurements andcalculation are done faster than the MPPT is working, there can be noneed to stop the MPPT on the inverter. Alternatively, when theenvironment is not stable, measurements can be taken few times for thesame radiation until a stable result is achieved.

Many variations can be applied to the systems and methods, withoutdeparting from the spirit presented in the disclosure. For example,additional components can be added, or components can be replaced. Forexample, rather than using a capacitor as primary energy store, aninductor can be used, or a combination of inductor and capacitor. Also,the balance between hardware and firmware in the micro controllers orprocessors can be changed. In some cases, only some problematic modulescan have a local management unit, for example in a shaded or partiallyshaded or otherwise different situation. In yet other cases, localmanagement units of strong modules can be virtually shut off. Themethods for determining the duty cycles for the solar modules can alsobe used to determine the duty cycles of groups of cells connected vialocal management units in a string within a solar panel/module.

FIG. 9 shows an overview of a local management unit (202 x) that ismodified from the local management unit (101) discussed above inrelation to FIG. 3A. In FIG. 9 , local management unit (202 x) containsa single chip micro controller (SCMC) (109). In one embodiment, all ofthe features and details of the local management units discussed aboveapply to the local management unit (202 x) and are not repeated forpurposes of clarity. In one embodiment, some of the features and detailsof the local management units discussed above selectively apply to thelocal management unit (202 x) and are not repeated for purposes ofclarity. The module driver (110) is connected in parallel with thecapacitor C1, and is also connected between the switches Q1 and Q2. Themicro controller (109) contains various operating parameters regardingthe local management unit (202 x), such as the voltage, current, etc.The micro controller (109) can run suitably programmed software (120a-n) to modulate the chopping frequency of the switches Q1 and Q2. Theswitches Q1 and Q2 perform a duty cycle according to the formulacalculated as previously described. A duty cycle would result in minorvariations from cycle to cycle (i.e., in the inter cycle) that can beused to encode using MFM (modified frequency modulation),Manchester-type encoding, or other suitable time-delay type encodingtechnique with or without additional error correction. As discussedfurther below, the approach of modulating, for example, the PWM intercycle would allow a receiver (301) at the end of the string bus (205) tomeasure the different variations of each of the local management units.Also, the local management units each can have a slightly different basefrequency so that their respective harmonics would not cover each other,although they would move in a similar range. This approach has the addedbenefit of reducing overall EMI of the system.

FIG. 10A is a plot of the upper half of a frequency spectrum (500) of acarrier frequency (501) for a particular local management unit. Thefrequency spectrum (500) shows the harmonics fn1-fnn as elements (505a-n). Arrows above the harmonics fn1-fnn (505 a-n) indicate they wobblearound with the variations in pulse width modulation from cycle tocycle. Also shown is a notch filter curve (504), which can be used toremove significant noise to avoid EMI problems in the system and tocomply with FCC and other regulatory agency regulations as needed.

FIG. 10B shows an overview of a subsystem (510) that includes the localmanagement unit (202 x), the panel voltage (180), terminals (182, 184,186), and a notch filter (506). In one embodiment, the notch filter(506) includes an inductor Ln and a capacitor Cn. The notch filter (506)acts as a low pass filter and relies on the internal capacity of thesingle chip micro controller (SCMC) of the local management unit (202x). A notch frequency of the notch filter (506) sits on the switchingfrequency to suppress noise. In one embodiment, additional or differentfilters can be used.

FIG. 11A shows an overview of a system (200) with a string bus (205)similar to that of system (200) previously discussed in relation toFIGS. 4A and 4B. In FIG. 11A, a receiver subsystem (300) is a receivingportion of a modem associated with a head end to receive modulatedsignals from local management units, as described in more detail below.The receiver subsystem (300) includes a receiving path separate from thestring bus (205) and the combiner box (206) so that the modulatedsignals from the local management units can be recovered beforeprovision to the combiner box (206) and significant noise therein. Thereceiver subsystem (300) includes a receiver (301), a sensing line(302), and a data output line (303). The sensing line (302) is connectedto the string bus (205) and the data output line (303) connects to thecombiner box (206). In one embodiment, the subsystem (300) can be insidethe inverter (203). In one embodiment, the subsystem (300) is containedin the combiner box (206). The subsystem (300) is shown external to thecombiner box (206) in FIG. 11A for purposes of clarity.

FIG. 11B shows the receiver (301). The receiver (301) includes a bandpass filter (310), a mixer (311), a beat oscillator (VCO) (312), amultiband pass filter (313), a microcontroller (314), and a power supply(315). Data from the local management unit arrives over the power bus205 via sensing line (302), and then passes through the band pass filter(310) to improve signal-to-noise ratio. The mixer (311) mixes the outputof the band pass filter (310) and the output of the VCO (312). Theoutput of the mixer (311) is then applied to the multiband pass filter(313), where the signal is analyzed in multiple band, frequency, andtime domains. The output of the multiband pass filter (313) is analyzedby the microcontroller (314). The power supply (315) can receive powerfrom the string bus (205) or from the inverter (203) and provide it tothe various elements of the receiver (301).

In one embodiment, the receiver (301) can manage communications from allthe local management units. In one embodiment, each local managementunit can have its own receiver. In one embodiment, a receiver can beimplemented in hardware (HW) only. In one embodiment, a digital radiocan be used as the receiver, in which case an analog to digitalconverter (ADC) samples the signals and all the processing is done in amicrocontroller or a digital signal processor using software (SW), orany combination of SW and HW.

FIG. 12 shows a novel topology of a local management unit (1200) as adistributed converter and remaining aspects of the local management unit(1200), as discussed above, are not shown for purposes of clarity. Inthe energy production or photovoltaic system, the local management unit(1200) in FIG. 12 can be used alternatively to the local managementunits discussed above. The local management unit (1200) is aseries-resonant converter with phase shift operation for light loadoperation. The local management unit (1200) includes capacitor Cin,switches Q1, Q2, Q3, Q4, inductor LR, capacitor CR, transformer having aprimary winding Tp coupled to a secondary winding Ts, diodes D1, D2, andtwo capacitors Cout. A typical range of input voltage Vin for the localmanagement unit (1200) is the standard panel voltage of Vmp plus orminus 20%. Output voltage Vout of the distributed converter is a fixedvalue of 375V plus or minus a few percentage points.

In operation, switch Q1 and switch Q2 are controlled oppositely, andswitch Q3 and switch Q4 are controlled oppositely. When switch Q1 is on,switch Q3 is on. When switch Q2 is on, switch Q4 is on. The current canbe increased or decreased by adjusting switches Q1, Q2, Q3, Q4. Acontroller (not shown), suitably connected to a power supply, controlsthe operation of the switches Q1, Q2, Q3, Q4. In one embodiment, thecontroller can be off the shelf and possibly modified. In oneembodiment, the controller can have analog circuitry. In one embodiment,the controller can be a microcontroller. In one embodiment, thecontroller could be a combination of these features. As discussed below,a phase shift can be created between the currents controlled by theswitches Q1, Q2, Q3, Q4. The inductor LR and the capacitor CR constitutean LC (or tank) circuit. The primary winding Tp of the transformer T iscoupled to the secondary winding Ts. Diode D1, diode D2, and capacitorCout constitute a Delon rectifier circuit. In a positive cycle, diode D1charges the upper capacitor of capacitor Cout. In a negative cycle,diode D2 charges the lower capacitor of the capacitor Cout. Vout iseffectively two times the voltage across the secondary winding Ts of thetransformer T.

The local management unit (1200) requires a reliable current limitbecause it is required to charge a large input capacitance reflectedfrom the inverter (203). The local management unit (1200) needs to allowoperation with low input and output capacitance, because reliabilitydoes not allow the use of aluminum capacitors due to their limited lifeexpectancy. In many instances aluminum may not be suitable for the localmanagement unit (1200) for reasons of reliability.

Efficiency of the novel topology of the local management unit (1200)should be higher than 96 percent at the range of 20 percent to 100percent load. The topology of the local management unit (1200) shouldallow direct control of input impedance for smooth MPPT control, andshould minimize the need for damping networks (i.e., snubbers) in orderto limit EMI emissions to improve reliability and maximize efficiency.Further, the transformer should be protected from saturation. Isolationvoltage must be higher than 2000V, and switching losses reduced (i.e.,zero current switching/zero voltage switching). No load condition is tobe defined during inverter turn on.

The local management unit (1200) achieves the aforementioned performancegoals. FIGS. 13 through 18 illustrate waveforms to show performance ofthe local management unit (1200) and the reduction of snub voltagetransients without resort to a snubber network in the local managementunit (1200). In FIG. 13 , waveform 1302 shows the current through theprimary winding Tp of the transformer T and waveform 1304 shows thedrain voltage at the switch Q1 at the MPPT point. The waveform 1304shows ringing on the square wave for only approximately two and a halfwaves at approximately one volt peak-to-peak.

In FIG. 14 , waveform 1402 shows the current through the primary windingTp of the transformer T and waveform 1404 shows the drain voltage at theswitch Q1 at 30 percent load.

FIG. 15 shows low input voltage at full load condition. In FIG. 15 ,waveform 1502 shows the current through the primary winding Tp of thetransformer T and waveform 1504 shows the drain voltage at the switch Q1at full load condition. Steps (1503) in the waveform 1502 result from aphase shift between switches. The steps (1503) results is reducedundershoot and overshoot in the waveform 1504.

FIG. 16 shows output diode voltage at resonant frequency at maximumload. In FIG. 16 , waveform 1602 shows the output current from the localmanagement unit (1200) to the inverter (203) and waveform 1604 showsdiode D1 (or diode D2) voltage at minimum frequency.

FIG. 17 shows typical output diode voltages at medium loads. In FIG. 17, waveform 1702 shows the output current from the local management unit(1200) to the inverter (203) and waveform 1704 shows diode D1 (or diodeD2) voltage at minimum frequency.

For loads higher than 15 percent of the maximum load, switches Q1, Q3are operated together at 50 percent duty cycle, while switches Q4, Q2are operated together at 50 percent duty cycle with no phase shift.Input power is controlled by changing operating frequency of the localmanagement unit (1200) above and below the resonant frequency. Turnratio of the primary winding Tp and secondary winding Ts is setaccording to MPPT voltage because at this voltage efficiency is at thehighest point (i.e., zero voltage, zero current is achieved). For otherfrequencies, switching is performed at zero voltage because there iscurrent in the primary winding Tp and resonant tank that is maintained,and this current causes voltage shift that allows turn-on to beperformed at zero voltage.

Below 15 percent of load, the local management unit (1200) is operatedin phase shift mode. In phase shift mode, switches Q1, Q2 are reversed,and switches Q3, Q4 are reversed. However, a phase shift causes switchesQ3 and Q4 to conduct together part of the time, and likewise forswitches Q1, Q4. A phase shift operation allows no load and light loadcontrol. As shown in, for example, FIG. 15 , steps 1503 in the waveform1502 are caused by the phase shift. The phase shift range and frequencyrange are optimized for maximum efficiency by the local management unit(1200). The switches (primary transistors) do not have off spike becausethey are clamped to the input bus. The phase shift minimizes ringing(and overshoot and undershoot), which in turn increases efficiency,reduces EMI, and reduces heat losses. Secondary diodes D1, D2 areconnected in center tap configuration to prevent voltage spikes fromdeveloping across them during turn-off and eliminating need for clampingcomponents.

As shown in FIG. 16 , a phase shift between the switches, as describedabove, causes a reduction in undershoot and overshoot in the diode D1voltage without implementation of snubber networks. As a result,efficiency of the local management unit (1200) is improved both on theswitch side and the diode side. In one embodiment, efficiency isimproved on each side by approximately 1-2%.

In the local management unit (1200), a resonant tank provides a limit tothe current through the primary winding Tp. A serial capacitor CRprevents transformer saturation. Output rectifier voltage is clamped tooutput voltage Vout allowing the use of 600V ultra fast diodes. Thereare no spikes across the switching transistors. Transformer parametersact as part of resonant tank. Input voltage range and efficiency areoptimized for solar module operation by transformer turn ratio andtransformer small air gap. Resonant frequency controls input impedance,which is the required control parameter for the application of separatesolar modules operating against a fixed voltage inverter load in thesystem.

FIG. 18 shows a spectral waveform (1802) of typical emissioncharacteristics of the local management unit (1200). Current ripple ofthe local management unit (1200) is measured with a current probe. Mostof the current ripple comes from the inverter (203). In one embodiment,the inverter (203) is an off the shelf item. From the spectral waveform(1802), it can be seen that data transmission is possible but needs tobe in the same level or higher level than the noise level. It can beseen that the maximum noise level value is approximately 35 dB.Switching frequency is clearly seen and can be detected in the spectralwaveform (1802).

FIG. 19 shows a local management unit (1900) that can be used inaccordance with one embodiment. The local management unit (1900) can beused in place of the local management units discussed above. The localmanagement unit (1900) includes a capacitor C1, switches Q1, Q2, diodeD1, inductor L, capacitor C2, controller 1902, terminals 1904, 1906,1908, and communication transmission modulator 1910. Operation of thelocal management unit (1900) is similar to the operation of the localmanagement units, as discussed above. Data transmission by the localmanagement unit (1900) involves modulating the switching frequency ofthe local management unit (1900) and transferring data by using thesolar module itself as power amplifier (PA).

Operation of the local management units in FIGS. 1-3A and FIG. 12involve pulse width modulation (PWM), as discussed above. The PWMtechnique creates noise, as shown in, for example, FIG. 18 . The creatednoise can be modulated to transmit data over the string bus (205) from asolar module (or slave node) to a head unit (master) in the energyproduction or photovoltaic system. The use of noise in this way avoidsthe need to provide a costly separate, dedicated communications channelfrom the solar module to the head unit.

The communication transmission modulator (1910) modulates switching ofthe pulse width modulation (PWM) operation to transmit data from thelocal management unit (1900). Various modulation encoding schemes can beused, such as, for example, modified FM (MFM) and Manchester coding. Inone embodiment, another modulating and encoding scheme can be used. Inone embodiment, the communication transmission modulator (1910)represents the transmission portion of a modem (not shown) that isassociated with the local management unit (1900). In one embodiment, thecommunication transmission modulator (1910) is part of the localmanagement unit (1900). In one embodiment, the communicationtransmission modulator (1910) is external to the local management unit(1900).

This system allows the use of full duplex (two-way) communications. Thereceiver at the module side can be implemented within the modulecircuitry. The limitation of transmit and receive within same circuitdoes not exist. Transmission from management unit can be used tosynchronize modules. Reliability is not affected by transmission. Theeffect on overall performance is very small because the transmissionduty cycle from the module is low.

Weak solar modules in a string bus, and weak string buses in a solararray can bring down the total output power of the solar array. A weakstring may produce less current, voltage, and/or power than otherstrings because one or more solar modules in the string are eitherpartially/wholly in shade or are malfunctioning (or for other reasons).Traditional solar arrays may not be able to overcome this problem sincethe output of individual solar modules and string buses may not becontrollable independently of other solar modules and string buses. Thesystems and methods herein disclosed monitor and adjust individual solarmodule outputs such that weak solar modules are balanced with strongsolar modules in a string bus, and strong string buses are balanced withweak string buses in a solar array. Once balancing within string busesand between string buses has been accomplished, an inverter usingmaximum power point tracking (MPPT) can determine the maximum powerpoint for the solar array.

Strong string buses are within a threshold voltage of a string outputvoltage of a strongest string, and weak string buses are not within thethreshold voltage of a string output voltage of a strongest string. Thestrongest string bus operates at a higher voltage than the other stringbuses. The weakest string bus operates at a lower voltage than the otherstring buses.

Three I-V curves for a string bus will now be used to describe balancingsolar modules on a string bus. FIG. 27 illustrates an I-V curve (2700)for a string bus where all solar modules (2702) are operating at theirideal outputs. There are six solar modules (2702) in the illustratedstring bus. A string bus I-V curve (2704) is derived from a composite ofall six ideal solar module I-V curves (2702). The voltage of the stringbus (2704) is derived by adding the voltages provided by each solarmodule (2702). The MPP (2706) is the point on the string bus I-V curve(2704) where current times voltage is maximized.

FIG. 28 illustrates an I-V curve (2800) for a string bus where two solarmodules are operating as weak solar modules. The weak solar modules(2808) produce the same voltage as the strong solar modules (2802), butcannot produce as large of currents. The string bus sees a loss ofcurrent (2812), since the string bus (2804) cannot produce the maximumcurrent of the strong solar modules (2802). The MPP (2806) is lower thanthat seen in the ideal string bus illustrated in FIG. 27 . In thisoperating region, the strong solar modules (2902) are operating atmaximum voltage, but are incapable of operating at their maximumcurrent. The string bus is thus producing less energy than what it iscapable of.

FIG. 29 illustrates an I-V curve (2900) for a string bus implementingthe systems and methods of this disclosure. The voltage of the two weaksolar modules (2908) can be decreased, which in turn increases theircurrent. The voltage can be decreased until the current balances withthe current from the strong solar modules (2902). This is what is meantby balancing solar modules on a string bus. As a result, the string busvoltage output decreases by a small value (2912) equal to the decreasein voltage for the two weak solar modules (2908). However, the stringbus current rises by a larger value (2914) to the level of what thestrong solar modules (2902) are capable of producing. The maximum powerproduced from the MPP (2906) of this I-V curve (2900) is greater thanthe maximum power produced when the weak solar modules are holding downthe current of the strong solar modules (FIG. 28 ). Stated differently,the sacrifice in voltage is more than compensated for by the increasedcurrent.

Three I-V curves for a solar array will now be used to describebalancing string buses in a solar array. FIG. 30 illustrates an I-Vcurve (3000) for a solar array where all string buses (3002) areoperating at their ideal outputs. There are nine string buses (3002) inthe illustrated solar array. A solar array I-V curve (3004) is derivedfrom a composite of all nine ideal string bus I-V curves (3002). Thecurrent of the solar array (3004) is derived by adding the currentsprovided by each string bus (3002). The MPP (3006) is the point on thesolar array I-V curve (3004) where current times voltage is maximized.

FIG. 31 illustrates a composite I-V curve (3100) for string buses in asolar array where two string buses are operating as weak string buses.The weak string buses (3108, 3110) produce the same current as thestrong string buses (3102), but cannot produce equivalent voltages. Thesolar array sees a loss of voltage (3112), since the solar array (3104)cannot produce the maximum voltage of the strong string buses (3102).The MPP (3106) is lower than that seen in the ideal solar arrayillustrated in FIG. 30 . In this operating region, the strong stringbuses (3102) are operating at maximum current, but are incapable ofoperating at their maximum voltage. The solar array is thus producingless energy than what it is capable of.

FIG. 32 illustrates an I-V curve (3200) for a solar array implementingthe systems and methods of this disclosure. Voltages of strong stringbuses (3202) can be decreased, which in turn increases the currentoutput from the strong string buses (3202). The voltages can bedecreased until all string bus voltages balance with the voltage of theweakest string bus (3208) or an average voltage of the weakest stringbuses (3208, 3210). This is one way to balance string buses in a solararray (another way will be described with reference to FIGS. 34-37 ). Asa result, the solar array voltage output decreases by a value (3112)equal to the decrease in voltage for the strong string buses (3202).However, the solar array gain in current (3214) is equal to the sum ofthe increased currents from all of the strong string buses (3202) whosevoltages were decreased. The maximum power produced from the MPP (3206)of this I-V curve (3200) is greater than the maximum power produced whenthe weak string buses (3208) were holding down the voltage of the strongstring buses (3202). Stated differently, the sacrifice in voltage ismore than compensated for by the increased current, even when conversionlosses are accounted for.

In an embodiment, the voltages of the strong string buses (3202) can bedecreased until the string bus voltage balances with the voltage of theone or more weak string buses (3208, 3210). Since the weak string buses(3208, 3210) may not operate at the same maximum voltage (e.g.,(3210)>(3208)), strong string buses (3202) can be balanced with anaverage of the weak string buses (3208), (3210).

It should be understood that FIGS. 27-32 are not drawn to scale, and areillustrative only. The curvatures of the I-V curves are also merelyillustrative and can vary significantly depending on the actual systemsthat the I-V curves represent. Although six solar modules and ninestring buses were described, these numbers are illustrative only. Anynumber of solar modules and string buses can be used.

Balancing solar modules in a string bus means that the solar modulecurrents converge. In an embodiment, balancing solar modules in a stringbus means that weak solar module currents converge on strong solarmodule currents. Balancing string buses in a solar array means thatstring bus voltages converge. In an embodiment, balancing string busesin a solar array means that strong string bus voltages converge on weakstring bus voltages. It should be understood that balancing need notmean an exact balance. Two values can only converge to within athreshold value of each other—a relative equivalency. For instance, twocurrents that are to be balanced by causing them to converge on 1 amp,can be considered balanced if they come within 0.1 amp of the 1 ampgoal. Two currents that are to be balanced by causing them to convergecan be considered balanced when they are within five percent of eachother. However, even when balanced, the process is iterative and willcontinue indefinitely. This is because the solar module performancechanges, the load changes, and local conditions (e.g., clouds, leaves,dirt, to name a few). Furthermore, every balancing of a string bus canrequire a balancing of solar modules on each string bus, and everybalancing of solar modules on each string bus can require a balancing ofstring buses.

FIG. 4B illustrates an embodiment of a solar array along with aninverter and a string combiner. In the illustrated embodiment the solararray 200 includes three string buses (205 a, 205 b, 205 c), althoughone or more string buses (205 a, 205 b, 205 c) can also be used. Thestring buses enable a series connection of solar modules (201 a, 201 b,201 n). Coupled between each solar module (201 a, 201 b, . . . 201 n)and its corresponding string bus (205 a, 205 b, 205 c), is a localmanagement unit (LMU) (202 a, 202 b, . . . , 202 n). The LMUs (202 a,202 b, . . . , 202 n) are controlled by a controller (204). Thecontroller (204) can communicate wirelessly with the LMUs (202 a, 202 b,. . . , 202 n) or via wireless repeaters. In an embodiment (notillustrated), wired connections between the controller (204) and theLMUs (202 a, 202 b, . . . , 202 n) can be implemented. String bus (205a, 205 b, 205 c) outputs are connected at an inverter (203) or in anoptional string combiner (206). The controller (204) can be configuredto balance current outputs from the solar modules on a string bus (205a, 205 b, 205 c). This can be done for each string bus (205 a, 205 b,205 c). Once, the current outputs from solar modules (201 a, 201 b, . .. , 201 n) on a string bus (205 a, 205 b, 205 c) are balanced (weaksolar module currents are raised to the level of strong solar modulecurrents within a string), the controller (204) can balance the currentoutputs from the string buses (205 a, 205 b, 205 c) (strong string busvoltages are lowered to the level of weak string bus voltages, which inturn raise strong string bus currents and hence the solar arraycurrent). This process can be repeated or an inverter (203) can thenattempt to determine the MPPT for the solar array (200).

A “solar array” typically comprises two or more solar modulesseries-connected via a string bus where the output voltage is a sum ofthe voltages of the series-connected solar modules. In larger solararrays, string buses can be connected in parallel such that theircurrents add. A combiner and inverter are not part of the solar array.

Balancing current outputs of solar modules (201 a, 201 b, . . . , 201 n)on a string bus (205 a, 205 b, 205 c) will now be discussed in moredepth. The controller (204) can be configured to balance the currentsproduced by the solar modules (201 a, 201 b, . . . , 201 n) on a givenstring bus (205 a, 205 b, 205 c), and perform this balancing for eachstring bus (205 a, 205 b, 205 c). As a result, the currents from thesolar modules (201 a, 201 b, . . . , 201 n) on a string bus (205 a, 205b, 205 c) can be balanced.

In order to balance solar modules (201 a, 201 b, . . . , 201 n) on astring bus (205 a, 205 b, 205 c), it can be useful to identify strongsolar modules (201 a, 201 b, . . . , 201 n) and weak solar modules (201a, 201 b, . . . , 201 n). This is done by varying the current on astring bus (205 a, 205 b, 205 c), monitoring the resulting change involtage in each solar module (201 a, 201 b, . . . , 201 n), andcomparing the changes in voltage on each solar module (201 a, 201 b, . .. , 201 n) to identify strong solar modules (201 a, 201 b, . . . , 201n) and weak solar modules (201 a, 201 b, . . . , 201 n).

Varying the current on the string bus (205 a, 205 b, 205 c) can involvethe inverter (203) pulling a different current from the string bus (205a, 205 b, 205 c). It can involve varying an impedance seen by the stringbus (205 a, 205 b, 205 c). For instance, the inverter (203) can vary theimpedance that the string bus (205 a, 205 b, 205 c) sees, and in doingso the current and voltage produced by the solar modules (201 a, 201 b,. . . , 201 n) on the string bus (205 a, 205 b, 205 c) will vary. Inother words, pulling a different current or changing the impedancechanges where on the I-V curve each solar module (201 a, 201 b, . . . ,201 n) operates at. Since current for devices connected in series is thesame, a change in current on the string bus (205 a, 205 b, 205 c) causesthe same change in current for each solar module (201 a, 201 b, . . . ,201 n) on the string bus (205 a, 205 b, 205 c). However, the changes involtage may not be the same, since the solar modules (201 a, 201 b, . .. , 201 n) can operate at different operating points on the I-V curve.

This can be seen in FIG. 22 , which illustrates an example of acomposite I-V curve (2203) for solar modules on a string bus. Thiscomposite I-V curve (2203) is not drawn to scale. Working points for twodifferent solar modules can be seen in FIG. 22 . The working point(2202) has a lower-angled slope and represents a weak solar module. Theworking point (2201) has a higher-angled slope and represents a strongsolar module. The variation in string bus current (2204 a) for the weaksolar module is the same as the variation in string bus current (2204 b)for the strong solar module since the solar modules are connected inseries, and thus must operate at the same current. However, since thetwo solar modules are at different working points on the I-V curve(2203), the resulting change in voltage (2206, 2205) for each is not thesame. The change in voltage dV2 (2206) for the weak solar module isgreater than the change in voltage dV1 (2205) for the strong solarmodule.

By identifying strong and weak solar modules, based on the changes involtage dV1, dV2, one can determine which solar module(s) (201 a, 201 b,. . . , 201 n) to adjust. Strong solar modules (201 a, 201 b, . . . ,201 n) can be used as a reference. Strong solar modules (201 a, 201 b, .. . , 201 n) may not be adjusted, while weak solar module (201 a, 201 b,. . . , 201 n) voltages can be decreased until their current outputsconverge on the strong solar module (201 a, 201 b, . . . , 201 n)outputs (or an average strong solar module (201 a, 201 b, . . . , 201 n)current output). This raises the current output of the string bus (205a, 205 b, 205 c), while decreasing the string bus (205 a, 205 b, 205 c)voltage output. However, the net effect is greater power output from thestring bus since the loss in voltage is more than compensated for by theincreased current. The end result can preferably be working points thatare proximal for all solar modules (201 a, 201 b, . . . , 201 n), thatis balanced or near balanced current outputs. An indication thatbalancing has been achieved and that the solar modules (201 a, 201 b, .. . , 201 n) are operating near the maximum current output of the strongsolar modules (201 a, 201 b, . . . , 201 n), is that a variation in thecurrent along the string bus (205 a, 205 b, 205 c) will cause a nearlyequivalent change in voltage for each solar module (201 a, 201 b, . . ., 201 n).

In an embodiment, instead of using strong solar modules (201 a, 201 b, .. . , 201 n) as the reference, an average of all solar modules (201 a,201 b, . . . , 201 n) on a string bus (205 a, 205 b, 205 c) can be usedas a reference. In this embodiment, all solar modules (201 a, 201 b, . .. , 201 n) on the string bus (205 a, 205 b, 205 c) can be adjusted,including strong solar modules (201 a, 201 b, . . . , 201 n), untiltheir current outputs converge on the average. In an embodiment, thecontroller (204) can identify strong and weak solar modules (201 a, 201b, . . . , 201 n) of all solar modules (201 a, 201 b, . . . , 201 n) ona string bus (205 a, 205 b, 205 c).

In an embodiment, the change in voltage dVi for each solar module (201a, 201 b, . . . , 201 n) can be checked for anomalies, and thosemeasurements appearing to be erroneous can be ignored or eliminated andreplaced with a new measurement of dVi. In an embodiment, at least onesolar module (201 a, 201 b, . . . , 201 n) can be identified as a strongsolar module (201 a, 201 b, . . . , 201 n). In an embodiment, the solarmodules (201 a, 201 b, . . . , 201 n) identified as strong solar modules(201 a, 201 b, . . . , 201 n) can be left out of the other stepsinvolved in balancing a string bus (205 a, 205 b, 205 c) (e.g., strongsolar module (201 a, 201 b, . . . , 201 n) current outputs may not bechanged while the current outputs of weak solar modules (201 a, 201 b, .. . , 201 n) are changed).

String buses (205 a, 205 b, 205 c) can be connected in parallel and havean output that is optionally connected to a string combiner (206) (orfuse box or chocks box). In an embodiment, the output of the stringbuses (205 a, 205 b, 205 c) can be connected to the inverter (203).

FIG. 21 illustrates exemplary solar module voltages for strong and weaksolar modules. Peaks and troughs are caused by changes in current on astring bus that the two solar modules are coupled to. Since the twosolar modules may not operate at the same working point on the I-Vcurve, the resulting changes in voltage dV1 (2101) and dV2 (2102) maynot be the same. Here, the stronger solar module has a smaller dV1(2101) since its working point corresponds to higher voltage and lowercurrent. The weaker solar module has a larger dV2 (2102) since itsworking point corresponds to lower voltage and higher current. Bymonitoring these voltage fluctuations, a controller or LMU can decreasethe voltage, and increase the current of the weak solar modules in orderto shift dV2 towards dV1.

In an embodiment, the LMUs (202 a, 202 b, . . . , 202 n) control thevoltage and current provided to the string buses (205 a, 205 b, 205 c)from the solar modules (201 a, 201 b, . . . 201 n). In an embodiment,one LMU can control the voltage and current output for more than onesolar module (201 a, 201 b, . . . , 201 n). In an embodiment, the numberof solar modules (201 a, 201 b, . . . , 201 n) can exceed the number ofLMUs (202 a, 202 b, 202 n). For instance, LMUs (202 a, 202 b, . . . ,202 n) can only be used to control the current and voltage output fromsolar modules (201 a, 201 b, . . . , 201 n) identified as weak solarmodules.

In an embodiment, a controller (204) can control the LMUs (202 a, 202 b,. . . , 202 n). The controller (204) can also monitor the string buses(205 a, 205 b, 205 c) and the solar modules (201 a, 201 b, . . . 201 n)via the LMUs (202 a, 202 b, . . . , 202 n). Data regarding the solarmodules (201 a, 201 b, . . . 201 n) and LMUs (202 a, 202 b, . . . , 202n) can be transmitted via the string buses (205 a, 205 b, 205 c) to thecontroller (204). In an embodiment, the controller (204) can transmitinstructions or commands to the LMUs (202 a, 202 b, . . . , 202 n) viathe string buses (205 a, 205 b, 205 c). In another embodiment, thecontroller (204) can perform the above-noted communications with theLMUs (202 a, 202 b, . . . , 202 n) via wireless communication paths. Thecontroller (204) can also be in communication with the inverter (203).In the illustrated embodiment, the controller (204) is a standalonedevice. However, in other embodiments, the controller (204) can be apart of other devices (e.g., the inverter (203), or LMUs (202 a, 202 b,. . . , 202 n). In an embodiment, the controller (204) can be a part ofone of the LMUs (202 a, 202 b, . . . , 202 n).

In an embodiment, operation of the controller (204) can be based onhistorical current and voltage data to help in pattern identification.For example, where the controller (204) notices that certain solarmodules (201 a, 201 b, . . . , 201 n) become weak solar modules (201 a,201 b, . . . , 201 n) at a specified time every day, this can be anindication of an object casting a predictable shadow over those solarmodules (201 a, 201 b, . . . , 201 n). As a result, instructions can besent to the affected LMUs (202 a, 202 b, . . . , 202 n) at the time whenthose LMUs (202 a, 202 b, . . . , 202 n) regularly become weak.

Balancing current outputs of string buses (205 a, 205 b, 205 c) will nowbe discussed in more depth. Although solar module (201 a, 201 b, . . .201 n) output current can be balanced on each string bus as describedabove, each string bus (205 a, 205 b, 205 c) can produce differentvoltages (i.e., weak string buses can produce less-than-ideal orless-than-maximum voltages).

Since string buses (205 a, 205 b, 205 c) can be connected in parallel,the voltages produced by the string buses (205 a, 205 b, 205 c) canconverge. This voltage convergence causes the working points of thesolar modules (201 a, 201 b, . . . 201 n) in each string bus (205 a, 205b, 205 c) to change. FIG. 23 shows exemplary plots of the resultingchange in current seen on two string buses when connected in parallel.The stronger string sees a decrease in current while the weak stringsees an increase in current.

In an embodiment, varying the voltage of the string buses in the solararray involves varying the current drawn from the string buses (205 a,205 b, 205 c) or varying an impedance seen by the string buses (205 a,205 b, 205 c). For instance, an inverter (203) connected to the stringbuses (205 a, 205 b, 205 c) can vary the impedance that the string buses(205 a, 205 b, 205 c) see, and in doing so the current and voltageproduced by the solar modules (201 a, 201 b, . . . 201 n) on the stringbus (205 a, 205 b, 205 c) will change. In other words, changing theimpedance changes where on the I-V curve each string bus (205 a, 205 b,205 c), and the solar modules (201 a, 201 b, . . . 201 n) on each stringbus, (205 a, 205 b, 205 c) operate at. A change in voltage on the stringbuses (205 a, 205 b, 205 c) causes a change in the current output fromeach of the string buses (205 a, 205 b, 205 c). However, since thestring buses (205 a, 205 b, 205 c) may not operate at the same operatingpoint on the I-V curve, the change in voltage will cause differingchanges in current for some or all of the string buses (205 a, 205 b,205 c).

This can be seen in FIG. 25 , which illustrates a composite I-V curve(2510) for string buses in the solar array (200). The I-V curve is notdrawn to scale. An operating point for two different string buses can beseen in FIG. 25 . Working point (2508) represents a weak string bus andworking point (2507) represent a strong string bus. The variation instring bus voltage dV1 (2512) is the same variation as seen for stringbus voltage variation dV2 (2511) since the string buses are connected inparallel. However, since the two string buses are at different workingpoints on the I-V curve (2510), the resulting change in current dI1(2503) and dI2, (2504) are not the same. The change in voltage dI2(2503) for the strong string bus is smaller than the change in voltagedI1 (2504) for the weak string bus.

By comparing the change in currents dI1, dI2 one can determine whichstring buses (205 a, 205 b, 205 c) to adjust. Adjusting string busvoltage output involves equally decreasing the voltage output of allsolar modules on a string bus (205 a, 205 b, 205 c), resulting in anincrease of the current from the string bus (205 a, 205 b, 205 c). In anembodiment, one or more weak string buses (205 a, 205 b, 205 c) can beused as references such that all other string bus voltages are balancedwith that of the one or more weak string buses (205 a, 205 b, 205 c). Inanother embodiment, an average of weak string buses (205 a, 205 b, 205c) can be used as the reference. In an embodiment, an average of allstring buses (205 a, 205 b, 205 c) can be used as the reference.

Not all solar modules can be adjusted. For instance, if two or morestring buses (205 a, 205 b, 205 c) are producing about the same current,then those string buses (205 a, 205 b, 205 c) can be used as references(the outputs from solar modules (201 a, 201 b, . . . 201 n) on thosestring buses (205 a, 205 b, 205 c) will not be changed). Solar moduleoutput currents on all other string buses (205 a, 205 b, 205 c) can besuch that the string bus output currents converge on the output from thereference string buses (205 a, 205 b, 205 c).

Having identified weak string buses (205 a, 205 b, 205 c), an averagechange in current dIw for the weak string buses (205 a, 205 b, 205 c)can be determined. The average change in current dIw for the weak stringbuses (205 a, 205 b, 205 c) can be a reference value. The change incurrent dIk for each strong string bus (205 a, 205 b, 205 c) can becompared to the average change in current dIw for the weak string buses(205 a, 205 b, 205 c). The difference between dIk for a string bus (205a, 205 b, 205 c) and dIw indicates by how much the string bus (205 a,205 b, 205 c) output current should be decreased in order to match theoutput current of the weak string buses (205 a, 205 b, 205 c) (to pushthe strong string bus (205 a, 205 b, 205 c) working point (2507) towardsthe weak string bus (205 a, 205 b, 205 c) working point (2508)).

For instance, and referring to FIG. 25 , the weak string bus (205 a, 205b, 205 c) change in current (2503) dI2 is greater than the strong stringbus (205 a, 205 b, 205 c) change in current (2504) dI1. Thus, the strongstring bus (205 a, 205 b, 205 c) current could be increased, which wouldreduce the voltage. The string bus (205 a, 205 b, 205 c) working point(2508) for the strong string bus (205 a, 205 b, 205 c) would thus shifttowards the working point (2507) of the weak string bus (205 a, 205 b,205 c). Similarly, the change in current dIk for each string bus (205 a,205 b, 205 c) can be compared to the average change in current of theweak string buses dIw and the current outputs for the strong stringbuses (205 a, 205 b, 205 c) adjusted such that all the string buses (205a, 205 b, 205 c) in the solar array (200) have working points thatconverge on those of the weak string buses. The end result canpreferably be working points that are proximal for all string buses (205a, 205 b, 205 c)—balanced or near balanced current outputs. Anindication that balancing has been achieved and that the string buses(205 a, 205 b, 205 c) are operating at nearly identical working pointson the I-V curve, is that a variation in the voltage for all stringbuses (205 a, 205 b, 205 c) will cause a nearly equivalent change incurrent for each string bus (205 a, 205 b, 205 c).

In an embodiment, before weak string bus (205 a, 205 b, 205 c) outputsare adjusted, one or more weak string buses (205 a, 205 b, 205 c) can bedisconnected from the other string buses (205 a, 205 b, 205 c) todetermine if disconnecting the one or more weak string buses (205 a, 205b, 205 c) increases the power output from the solar array (200).

The inverter (203) can convert the direct current (DC) outputs from thestring buses (205 a, 205 b, 205 c) to an alternating current (AC) outputthat can be supplied, for example to a power grid or other load (e.g., ahome or business). The inverter (203) can control the current andvoltage drawn from the string buses (205 a, 205 b, 205 c), and thuscontrol where on the I-V curve the string buses (205 a, 205 b, 205 c)operate at. For instance, as the inverter (203) can increase impedanceseen by the solar array (200), which will cause the current drawn fromthe string buses (205 a, 205 b, 205 c) to decrease and the voltage toincrease, and the working point will shift along the I-V curve to theright towards where the I-V curve meets the x-axis (voltage). Thus, ifthe string buses (205 a, 205 b, 205 c) are operating at a working pointhaving a higher current and lower voltage than the MPP for the solararray (200), then the inverter (203) can increase the impedance causingthe string bus (205 a, 205 b, 205 c) working points to shift towards theMPP. Balancing can be carried out via the methods described withreference to FIG. 26 .

FIG. 20 illustrates an exemplary inverter current controlled by amaximum power point tracking algorithm. Such a current can occur when asolar array is connected to an MPPT-enabled inverter such as, but notlimited to, SB300 made by SMA America, Inc., and IG2000 made by FroniusUSA, LLC. A typical MPPT algorithm pulls and pushes the current (2003)in the solar array (for example, by increasing and decreasing theimpedance seen by the solar array) causing I to fluctuate into peakcurrent spikes (2001 a-n) and valley current spikes (2002 a-n). Thesefluctuations cause solar module voltage fluctuations. These fluctuationscan be used to determine the MPP for solar modules.

FIG. 24 illustrates an exemplary current versus time diagram for astronger and weaker string bus when the voltage to the string busesfluctuates. The stronger string bus has a smaller current change (2401)since the working point is closer to the closed-circuit current (wherethe I-V curve meets the Y-axis). The weaker string bus has a largercurrent change (2402) since the working point is closer to theopen-circuit voltage (where the I-V curve meets the X-axis).

FIG. 26 illustrates an embodiment of a method (2600) of maximizing thepower output of a solar array by (1) balancing current outputs of solarmodules, (2) balancing voltage outputs of string buses, and (3) applyingan MPPT algorithm to the solar array. The method (2600) can alsomaximize power by identifying strong and weak solar modules beforebalancing current outputs of solar modules and by identifying strong andweak string buses before balancing voltage outputs of string buses. Themethod (2600) can be carried out via an optional first identifyoperation (2602), a first balance operation (2604), an optional secondidentify operation (2612), a second balance operation (2614), and anapply operation (2620).

The order that balancing operations and the applying operation occur incan vary. For instance, currents produced by solar modules in each ofone or more string buses can be balanced in a first balance operation(2604). Voltages produced by the one or more string buses can then bebalanced in a second balance operation (2614). An MPPT algorithm canthen be applied to the solar array in an apply operation (2620). Inanother embodiment, the method (2600) can begin with the first balanceoperation (2604) followed by one or more loops of the apply operation(2620). The second balance operation (2614) can then occur followed byone or more further loops of the apply operation (2620). It should beunderstood that the first and second balance operations (2604, 2614) andthe apply operation (2620) can operate in any order or pattern, witheach operation repeating one or more times before another operationoperates. The first and second balance operation (2604, 2614) can pausebetween operations to allow the apply operation (2620) to repeatnumerous times. In an embodiment, the balance operations (2604, 2614)can run simultaneously with the apply operation (2620). In anembodiment, the applying an MPPT algorithm is not a part of the method(2600). Rather, the method (2600) can merely include balancing the solarmodules and the string buses.

As noted above, the method (2600) includes the balance currents producedby solar modules in each of one or more string buses operation (2604).The solar modules can be connected by a string bus of the solar array,for instance in series. In an embodiment, balancing involves (1) varyingthe current on the string bus, (2) monitoring changes in voltage outputfor each solar module on the string bus, (3) comparing the monitoredchanges in voltage output for the solar modules on the string bus, and(4) adjusting the current output of one or more of the solar modulessuch that the current outputs from all solar modules on the string busconverge (see FIG. 29 ).

In an embodiment, before the first balance operation (2604), strong andweak solar modules can be identified via a first identify operation(2602). The first identify operation (2602) identifies, for each stringbus, one or more strong solar modules and one or more weak solarmodules. Weak solar modules can be adjusted such that their outputsconverge on those of the strong solar modules.

In an embodiment, the first identify operation (2602) can be carried outby: (1) varying the current on the string bus (or impedance seen by thestring bus), (2) monitoring changes in voltage output for each solarmodule, and (3) comparing the changes in voltage output. Strong solarmodules can be those having the smallest change in voltage. Strong solarmodules can also be characterized as operating at relatively the samecurrent (indicated by similar changes in voltage). Strong solar modulescan be those having working points furthest to the right on the I-Vcurve (e.g., working point (2201) in FIG. 22 ).

Having identified strong solar modules, an average change in voltage dVsfor the strong solar modules can be determined. The average change involtage dVs for the strong solar modules can be a reference value. Thechange in voltage dVi for each solar module can be compared to theaverage change in voltage dVs for the strong solar modules. Thedifference between dVi for a solar module and dVs indicates by how muchthe solar module output current should be decreased in order to matchthe output current of the strong solar modules (to push the weak solarmodule working point (2202) towards the working point (2201) of thestrong solar modules).

In an embodiment, once the first identify and balance operations (2602,2604) have been performed, strong and weak solar modules can again beidentified via the first identify operation (2602). This can be donesince some strong solar modules can have become weak while the weaksolar modules were being adjusted. If a weak solar module is identifiedthat was previously a strong solar module, then its current output canbe adjusted in the next loop of the first balance operation (2604).

The method (2600) can also include a second balance operation (2614). Inan embodiment, the second balance operation (2614) includes (1) varyingthe voltage of the string buses in the solar array, (2) monitoringchanges in current of each string bus, (3) comparing the monitoredchanges in current, and (4) adjusting the voltage output of the stringbuses (by changing the voltage output of all solar modules in a stringbus) such that the string bus output voltages converge (see FIG. 32 ).

In an embodiment, before the second balance operation (2614), strong andweak string buses can be identified via a second identify operation(2612). The second identify operation (2612) identifies strong and weakstring buses in the solar array. Strong string buses can be adjustedsuch that their outputs converge on those of the weak string buses. Inan embodiment, the second identify operation (2612) can be carried outby: (1) varying impedance seen by the string buses (or the voltage onthe string buses), (2) monitoring changes in current output for eachstring bus, and (3) comparing the changes in current output. Weak stringbuses can be those having the smallest change in current. Weak stringbuses can also be characterized as operating at relatively the samecurrent (indicated by similar changes in current). Weak string buses canbe those having working points furthest towards the left of the I-Vcurve (e.g., working point (2507) in FIG. 25 ).

The method (2600) also includes an apply operation (2620) wherein themaximum power point tracking algorithm is applied. Maximum power pointtracking (MPPT) is a procedure or algorithm used to determine themaximum power point (MPP) of a system—in this case the maximum powerpoint of a solar array (see, e.g., “Maximum power” in FIG. 22 andworking point (2506) in FIG. 25 ). In other words, an MPPT algorithm canadjust the current and voltage produced by the solar array until thevoltage times current is maximized. A variety of algorithms and devicescan be used to carry out MPPT. For instance, in an embodiment, aninverter connected to an output of the solar array can change theimpedance that the solar array sees, thus causing the voltage andcurrent produced by the solar array to change. By sweeping or flutteringthe impedance over one or more ranges of values, an MPPT algorithm candetermine what impedance corresponds with the MPP, and set the impedanceto that value such that the solar array operates at the MPP. In anotherembodiment, MPPT can include the steps of (1) adjusting the impedancethat the solar array sees (or adjusting the solar array voltage orcausing the working point to move along the I-V curve), (2) monitoringthe solar array's reaction to the adjusting impedance (or current orworking point), (3) continuing to adjust the impedance (or current orworking point) and monitoring the solar array's response, (4) based onthe monitoring, determine a maximum power point for the solar array, and(5) set the impedance (or current or working point) to a value thatcorresponds to the solar array's MPP.

The apply operation (2620) can operate after either of the balanceoperations (2604, 2614). The apply operation (2620) can also operateafter any number of repetitions or loops of either or both of thebalance operations (2604, 2614). In an embodiment, the apply operation(2620) can operate after any number of repetitions or loops of the firstidentify operation (2602) and the first balance operation (2604); afterany number of repetitions or loops of the second identify operation(2612) and the second balance operation (2614); and after any number ofrepetitions or loops of a combination of the first and second identifyand apply operations (2602, 2604, 2612, 2614). For instance, strong andweak solar modules in each of one or more string buses can be identifiedin the first identify operation (2602). Currents produced by solarmodules in each of the one or more string buses can be balanced in thefirst balance operation (2604). Strong and weak string buses in each ofthe one or more string buses can be identified in the second identifyoperation (2612). Voltages produced by the one or more string buses canthen be balanced in the second balance operation (2614). An MPPTalgorithm can then be applied to the solar array in the apply operation(2620).

In another embodiment, the method (2600) can begin with the firstidentify operation (2602) and the first balance operation (2604)followed by one or more loops of the apply operation (2620). The secondidentify operation (2612) and the second balance operation (2614) canthen occur followed by one or more further loops of the apply operation(2620). It should be understood that the first and second identifyoperation (2602, 2612), the first and second balance operations (2604,2614), and the apply operation (2620) can operate in any order orpattern, with each operation repeating one or more times. In anembodiment, the apply operation (2620) can operate multiple times beforethe first or second identify operations (2602, 2612) resume.Alternatively, the identify and balance operations (2602, 2604, 2612,2614) can run simultaneously with the apply operation (2620). In anembodiment, the applying an MPPT algorithm is not a necessary part ofthe method (2600). Rather, the method (2600) can merely includebalancing the solar modules and the string buses.

In this disclosure, systems and methods for balancing string buses havebeen described where string bus output voltages for strong string buseswere decreased until they converged on weak string bus voltages (seeFIG. 32 ) resulting in an increased current from the solar array.However, string bus output voltage can also be balanced by increasingstring bus output voltages, as discussed below.

FIG. 33 illustrates an I-V curve (3300) for a solar array having stringbuses balanced via increasing string bus output voltage. Recall thatFIG. 31 illustrates the I-V curve (3100) for the solar array before thestring bus output voltages are balanced. FIG. 32 illustrates the I-Vcurve (3100) for the solar array after the string bus output voltagesare balanced by decreasing the voltage output of strong string buses tomatch that of weak string buses. In contrast, in FIG. 33 the voltageoutputs of the two weak string buses (3308, 3310) are increased tobalance them with the voltage outputs of the strong string buses (3302).As such, the voltage output for the solar array (3304) increases by again in voltage (3312). The solar array voltage before balancing isrepresented by the vertical dashed line. As a result of increasing thevoltage output from the weak string buses (3308, 3310), the currentoutput of the weak string buses (3308, 3310) decreases causing the solararray current output to decrease by a loss in current (3314). However,the gain in voltage along with benefit of avoiding losses associatedwith downconverting the strong string bus output voltages, more thancompensates for the loss in current (3314).

FIG. 34 illustrates an embodiment of a solar array (3400) according tothe present disclosure. Like previously described solar arrays in thisdisclosure, the solar array (3400) includes two strings (3410, 3420) (orstring buses), although two or more can be used. For the purposes ofclarity only one string (3410) will be described. However, both strings(3410, 3420) can have identical or similar structures, so thisdiscussion also can apply to the second string (3420) in one embodiment.The string includes two or more local management units (LMUs) (3412) andtwo or more solar modules (3411). Each LMU (3412) is connected inparallel with one of the solar modules (3411). The LMU (3412) controlsthe current on the string (3410) by downconverting a voltage provided bythe solar modules (3411). The string output voltage (3414) is the sum ofthe voltages provided by the solar modules (3411) on the string (3410).The LMUs (3412) are connected in series via conductive connections(e.g., wires, leads, power cables). The string (3410) collects powerfrom the solar modules (3411) and conducts the power towards a combiner,inverter, or load (3430). The strings (3410, 3420) can be connected inparallel via a parallel bus (3450). Each string (3410, 3420) has astring output voltage (3414, 3424). In an embodiment, one or more solarmodules (3411) can be connected directly to the string (3410) without anLMU (3412). In a further embodiment, each string can be replaced with asingle solar panel.

Unlike solar arrays previously discussed in this disclosure (recallFIGS. 4A-4B), the solar array (3400) also includes local stringmanagement units (LSMUs) (3413, 3423). The LSMUs (3413, 3423) produce asolar array output voltage (3415) and are connected to a combiner (orcombiner box) (3430), an inverter (3430), or a load (3430). If connectedto an inverter (3430), then the inverter (3430) can run an MPPTalgorithm to find the MPP for the solar array (3400) after every loop ofbalancing (of both solar modules (3411, 3421) and strings (3410, 3420)).A first LSMU (3413) is connected to the first string (3410) and isconfigured to convert the first string output voltage (3414) to thesolar array output voltage (3415). A second LSMU (3423) is connected tothe second string (3420) and is configured to convert the second stringoutput voltage (3424) to the solar array output voltage (3415). TheLSMUs (3413, 3423) are connected between positive and negative ends ofthe strings (3410, 3420) as illustrated in FIG. 34 . The LSMUs (3413,3423) can be connected in parallel via parallel bus (3450). The LSMUs(3413, 3423) are configured to upconvert or downconvert the stringoutput voltages (3414, 3424) for the parallel bus 3450. The LSMUs (3413,3423) can also monitor the string output voltages (3414, 3424) and thesolar array output voltage (3415). The LSMUs (3413, 3423) can alsotransmit data regarding the string output voltages (3414, 3424) or thesolar array output voltage (3415) to each other via an optionalconnection 3442, which can be a wired or wireless connection. The LSMUs(3413, 3423) can be configured to identify a strongest string and aweakest string. In an embodiment, the strongest string has a largerstring output voltage (3414, 3424) than all other strings (3410, 3420).In an alternative embodiment, the strongest string produces more powerthan all other strings (3410, 3420). The weakest string has a smallerstring output voltage (3414, 3424) than all other strings (3410, 3420).In an alternative embodiment, the weakest string produces less powerthan all other strings (3410, 3420). The LSMUs (3413, 3423) can beconfigured to identify strong strings—strings that are within a rangefrom the output voltage (or power) of a strongest string. The LSMUs(3413, 3423) can be configured to identify weak strings—strings that arenot within a range from the output voltage (or power) of a strongeststring. For instance, if the strongest string has a string outputvoltage of 5V and the range is 0.2V, then all strings having a stringoutput voltage of 4.8V-5.0V will be identified as strong strings. Allother strings will be identified as weak strings. The LSMUs (3413, 3423)can be configured to select a target voltage. The target voltage is thevoltage at which the LSMUs (3413, 3423) push the solar array outputvoltage (3415) towards by either upconverting or downconverting thestring output voltages (3414, 3424).

In an embodiment, balancing strings (3410, 3420) using LSMUs (3413,3423) can entail first identifying the strongest string. In anembodiment, the LSMUs (3413, 3423) can be configured to identify strings(3410, 3420) that are within the range from the output voltage of thestrongest string and to identify strings that are not within the rangefrom the output voltage of the strongest string.

In an embodiment, one LSMU (3413, 3423) can be a controlling LSMU. Thecontrolling LSMU can do any one or more of the following: monitor thestring output voltages (3414, 3424); transmit data regarding the stringoutput voltages (3414, 3424) to other LSMUs (3413, 3423); monitor thesolar array output voltage (3415); transmit data regarding the solararray output voltage (3415) to other LSMUs (3413, 3423); identify thestrongest string; identify the weakest string; identify strings that arewithin the range from the output voltage of the strongest string;identify strings that are not within the range from the output voltageof the strongest string; and select the target voltage.

Optionally, a management unit (3440) can perform the tasks of thecontrolling LSMU. The management unit (3440) can be in wired or wirelesscommunication with the LSMUs (3413, 3423). The management unit (3440)can monitor the string output voltages (3414, 3424), as well as thesolar array output voltage (3415), via optional connection (3442). Themanagement unit (3440) can identify a strongest string and a weakeststring along with strings that are within the range from the outputvoltage of the strongest string, and strings that are not within therange from the output voltage of the strongest string. The managementunit (3440) can select the target voltage. The management unit (3440)can communicate with other management units. Those other managementunits can be in communication with other solar arrays. The othermanagement units can be monitor other solar arrays and thus be externalto the solar array 3400.

In an embodiment, balancing strings (3410, 3420) involves converting thestring output voltages (3414, 3424) to the target voltage on theparallel bus (3450). In an embodiment, the target voltage is selected tobe equal to a string output voltage (3414, 3424) of the strongeststring. In this case, string output voltages (3414, 3424) for strings(3410, 3420) that are not within the range from the output voltage ofthe strongest string can be upconverted and/or downconverted so that thesolar array output voltage (3415) of the parallel bus (3450) equals thestring output voltage (3414, 3424) of the strongest string. Since thestring output voltage (3414, 3424) for the strongest string is equal tothe target voltage, the LSMU (e.g., 3413 or 3423) connected to thestrongest string (and other LSMUs connected to strings having outputvoltages within the range of the voltage output of the strongest string,if any) can operate in a bypass mode. In bypass mode, a converter of theLSMU (e.g., 3413, 3423) is turned off to reduce power consumption.Current can pass around or through the LSMU (3413, 3423) while losing nomore energy than is lost in normal transit through low-loss or lowimpedance wires or circuits. In an embodiment, current passes throughlow-loss portions of the LSMU (3413, 3423). In an embodiment, the bypassmode involves routing current through a transistor, for instance aMOSFET or bipolar transistor.

In an embodiment, the target voltage is equal to an average stringoutput voltage of strings (3410, 3420) that are within the range fromthe output voltage of the strongest string. In this case an averagestring output voltage (3414, 3424) can be determined for the strings(3414, 3424) that are within the range from the output voltage of thestrongest string where the average includes the voltage of the strongeststring. String output voltages (3414, 3424) of strings (3410, 3420) thatare not within the range from the output voltage of the strongest stringare upconverted so that the solar array output voltage (3415) equals theaverage string output voltage of the strings (3410, 3420) that arewithin the range from the output voltage of the strongest string. Onlystring bus output voltages (3414, 3424) for the strings (3410, 3420)that are not within the range from the output voltage of the strongeststring are upconverted. LSMUs (3413, 3423) connected to the strings(3410, 3420) that are within the range from the output voltage of thestrongest string, (including the strongest string) operate in bypassmode.

In an embodiment, the target voltage is less than the average outputvoltage of the strings (3410, 3420) that are within the range from theoutput voltage of the strongest string. Strings (3410, 3420) havingstring output voltages (3414, 3424) that are less than the averageoutput voltage of the strings (3410, 3420) that are within the rangefrom the output voltage of the strongest string are upconverted so thatthe solar array output voltage (3415) equals the target voltage. Strings(3410, 3420) having string output voltages (3414, 3424) that are greaterthan the average output voltage of the strings (3410, 3420) that arewithin the range from the output voltage of the strongest string aredownconverted so that the solar array output voltage (3415) equals thetarget voltage. In this embodiment, some LSMUs (3413, 3423) areconfigured to both upconvert and downconvert voltages.

In an embodiment, the target voltage is greater than the string outputvoltage (3414, 3424) of the strongest string. In this case string outputvoltages (3414, 3424) for all strings (3410, 3420), including thestrongest string, are upconverted so that the solar array output voltage(3415) equals the target voltage.

In one embodiment of a bypass mode of an LSMU (3413, 3423), theconverter of the LSMU (3413, 3423) is not used, and a conductive patharound the converter of the LSMU (3413, 3423) is used to bypass theconverter of the LSMU (3413, 3423). In other words, when an LSMU (3413,3423) is in bypass mode, current travels from a string (3410, 3420) tothe inverter/combiner/load (3430) with a low voltage loss. For instance,a transistor can route current around the voltage converter of the LSMU(3413, 3423). For instance, a bypass switch (e.g., transistor) could beused to redirect current through a low-resistance wire in the LSMU(3413, 3423) rather than through the circuitry that causes losses. In anembodiment, such a switch can route current around the outside of theLSMU (3413, 3423). In an embodiment, the bypass mode can be the modethat the LSMU (3413, 3423) normally operates in unless instructedotherwise.

In an embodiment, the solar array (3400) can be connected to a combiner,inverter, or load (3430). For the case of a combiner (3430), the LSMUs(3413, 3423) can be connected in parallel via the combiner (3430). Thisconnection can exist within or outside of the combiner (3430). In anembodiment, there can be one to four LSMUs (3413, 3423) in a combiner.However, it is also possible to have more than four LSMUs (3413, 3423)combined in a combiner (3430). The output of the combiner (3430) can beprovided to an inverter (not shown). In an embodiment, outputs frommultiple combiners (3430) can be combined in parallel and provided to aninverter. In one embodiment, the LSMUs (3413, 3423) and at least aportion of the parallel bus (3450) are located within the combiner box(3430).

In an embodiment, the solar array voltage output (3415) can be providedto a load (3430). Non-limiting examples of loads include computers, cellphones, single or multi-family dwellings, apartment or commercialstructures, and rechargeable vehicles, to name a few.

In an embodiment, the management unit (3440) and/or the LSMUs (3413,3423) can reside on or as part of the solar array (3400). In anembodiment, the management unit (3440) and/or the LSMUs (3413, 3423) canreside on one or more solar modules (3411, 3421). In an embodiment, themanagement unit (3440) and/or the LSMUs (3413, 3423) can reside outsidethe solar array (3400).

In an embodiment, the string output voltages of two more strong stringscan be used to select the target voltage, when the string outputvoltages of these strong strings are within a predetermined range. Theselected target voltage allows the LSMUs of these strong strings tooperate in a bypass mode to reduce power consumption and/or conversionlosses by the LSMUs. In one embodiment, there may be multiple groups ofstrong strings that have voltages within the predetermined range, andthe group that can achieve a greatest reduction in power consumption viaoperating in a bypass mode is used to select the target voltage. Thestring output voltages of other strings are upconverted and/ordownconverted to the target voltage. For example, two second strongeststrings may have a lower string output voltage or a lower power outputthan the strongest string, but have a larger string output voltage or ahigher power output than all other strings. When the two secondstrongest strings are used to select the target voltage they can beoperated in the bypass mode. In one embodiment, when the two secondstrongest strings operate in bypass mode, the solar array output powercan be greater than the solar array output power produced when only thestrongest string is operated in bypass mode.

In an embodiment, the strings can be dived into a first group of stringsand a second group of stings. The first group of strings have a greaterstring output voltage, or greater string output power, than the secondgroup of strings. The first group of strings have string output voltagesor power that are within a range of each other such that when connectedto the parallel bus, their string output voltage or power is notdegraded (or the degradation is less than operating them in non-bypassmode). In some embodiments, some of the strings in the second group maybe stronger than the strings in the first group. The first group ofstrings is selected to operate in bypass mode to increase the poweroutput of the array. For example, a first amount of power may be lostwhen a string is connected to the parallel bus via an LSMU in bypassmode, in which the string (and other strings connected to the parallelbus in bypass mode) might move away from the maximum power point becauseof the influence of the parallel bus; and a second amount of power maybe lost when the string is connected to the parallel bus via the LSMUupconverting or downconverting the output voltage of the string. Whenthe LSMU is not in the bypass mode, the LSMU can isolate the string fromthe parallel bus and thus allow the string to run on or near the maximumpower point. However, the operation of upconverting or downconvertingconsumes power. When the first amount of power is less than the secondamount of power, the string is selected to be part of the first groupfor bypass mode; and when the second amount of power is less than thefirst amount of power, the string is selected to be part of the secondgroup for non-bypass mode.

FIG. 35 illustrates an embodiment of an LSMU (3500). The LSMU (3500) canbe configured to up-convert and/or down-convert an input voltage to atarget voltage. The LSMU (3500) can include an input (3502) configuredto receive an input voltage from the string. The LSMU (3500) can includea voltage conversion portion (3508) connected to the input (3502). Thevoltage conversion portion (3508) can be configured to convert the inputvoltage to the target voltage. The voltage conversion portion (3508) canbe configured to operate in a bypass mode. The LSMU (3500) can includean output (3518) connected to the voltage conversion portion (3508). Theoutput (3518) can be configured to provide the target voltage to acombiner, inverter, and/or load. For instance, the output (3518) canprovide the target voltage to a combiner to be combined with outputvoltages from other strings. A combined output voltage from the combinercan then be provided to an inverter and from there provided to a load orthe grid.

As noted, the LSMU (3500) can include the input (3502). The input (3502)can be configured to receive an input voltage from the string. The input(3502) is a portion of the LSMU (3500) capable of forming a conductivepath with other electrical devices. For instance, the input (3502) canbe connected to the string. In an embodiment, the input (3502) can be aconnector or a terminal. The input (3502) can have two poles, onepositive and one negative. The negative pole can be grounded.

As noted, the LSMU (3500) can include the voltage conversion portion(3508). The voltage conversion portion (3508) can be configured toconvert the input voltage to a target voltage. In other words, thevoltage conversion portion (3508) can be a transformer or anyvoltage-converting device or circuit (e.g., Buck, Buck-Boost, and Cukconverters, to name a few). In one embodiment, the voltage conversionportion (3508) is configured to upconvert the input voltage. In anotherembodiment, the voltage conversion portion (3508) is configured todownconvert the input voltage. In a further embodiment, the voltageconversion portion (3508) can be configured to either upconvert ordownconvert the input voltage, based on a control signal.

In one embodiment, the target voltage for the parallel bus (3450) isdetermined to increase the total power output from the strings (3410,3420) to the parallel bus (3450). In the one embodiment, an LSMU isplaced in the bypass mode to increase the total energy output of thesolar array. For example, if the power consumption of the converter ofan LSMU operated in a voltage-converting mode is larger than the drop ofpower output caused by operating the LSMU in a bypass mode, then theLSMU can be switched to the bypass mode to increase overall poweroutput; otherwise, the LSMU can be switched to the voltage-convertingmode.

The voltage conversion portion (3508) can include multiple systems, oneof which is used to upconvert the input voltage and the other is used todownconvert the input voltage. The LSMU (3500) or the voltage conversionportion (3508) can be configured to operate in a bypass mode. Thevoltage conversion portion (3508) can cause 1-2% energy loss when it isdownconverting or upconverting the input voltage. Thus, when the stringthat the LSMU (3500) is connected to is identified as a string having astring output voltage within a range from the output voltage of thestrongest string voltage of a string output voltage of a strongeststring, and the target voltage is within a range from the output voltageof a strongest string, the LSMU (3500) can avoid the 1-2% energy loss byoperating in the bypass mode.

As noted previously, the bypass mode is a low-loss mode wherein currentis routed through a low-loss conductive path. For instance, theswitchable connection (e.g., transistor) (3506) can be used to routecurrent around the voltage conversion portion (3508). This allowscurrent to pass from the string, through the LSMU (3500), and to thecombiner, inverter, or load while avoiding the losses of travelingthrough the voltage conversion portion (3508). The voltage conversionportion (3508) can have a connection to ground.

A local controller (3504) can control the switchable connection (3506).For instance, if the switchable connection (3506) is a transistor, thenthe local controller (3504) can control the transistor (3506) gate. Thelocal controller (3504) can be implemented in hardware, software, or acombination of the two. In an embodiment, a diode can be connected inparallel to the transistor (3506), or used to replace the transistor(3506) in another embodiment.

As noted, the LSMU (3500) provides the output (3518). The output (3518)can be configured to provide the target voltage to a combiner, inverter,or load. The output (3518) is a portion of the LSMU (3500) capable offorming a conductive path with other electrical devices. For instance,the output (3518) can be connected to an inverter or load. Non-limitingexamples of the output (3518) include a connector or terminal, to nametwo. The output (3518) can be connected to outputs from other LSMUs. Inan embodiment, outputs of different LSMUs can be combined within acombiner. Connections between LSMU outputs can be made in parallel. Inan embodiment, the output (3518) can provide a fixed voltage invertervoltage to an inverter (or to a combiner and then to an inverter). Theoutput (3518) can have two poles, one positive and one negative. Thenegative pole can be grounded.

FIG. 36 illustrates another embodiment of a local string management unit(3600). The LSMU (3600) is similar to the LSMU (3500), but providesdetails as to possible components that make up the voltage conversionportion (3508). In the illustrated embodiment, such components functionto convert and filter the input voltage. Like the LSMU (3500), the LSMU(3600) includes an input (3602), an output (3618), and can include aswitchable connection (3606) for enabling current to bypass the voltageconversion circuitry.

Unlike LSMU (3500), LSMU (3600) can include an input filter capacitor(3614) and can include an output filter capacitor (3616). The LSMU(3600) also can include a diode (3610) or rectifier. The LSMU (3600) caninclude a local controller (3604), which can be implemented in hardware,software, or a combination of the two. The local controller (3604) canbe in communication with the management unit (3440) discussed withreference to FIG. 34 .

In one embodiment, the LSMU (3600) includes a booster converter whichincludes an inductor (3608) and a switchable connection (3612). Theoutput voltage of the boost converter is determined by the duty cycle ofthe switch (3612). In the bypass mode, when the switchable connection(3606) is closed, losses due to current passing through the inductor(3612) and the diode (3610) are reduced (and/or are negligible). Thusthe ratio of input voltage to target voltage can be controlled via thelocal controller (3604). The local controller (3604) can also controlthe bypass mode by controlling the switchable connection (3606). TheLSMU (3600) can include a connection (3620) able to connect to otherLSMUs. For instance, the connection (3620) can allow LSMUs to beconnected within a combiner box.

FIG. 37 illustrates an embodiment of a method for balancing strings in asolar array using LSMUs. The method (3700) can include a selectoperation (3702), an operate operation (3704), and an instruct operation(3706).

The select operation (3702) can include selecting the output voltage ofa string having a string output voltage within a range from the outputvoltage of a strongest string as a target voltage. A string having astring output voltage within a range from the output voltage of astrongest string can be referred to as a “strong string.” Alternatively,the select operation (3702) can include selecting the average stringoutput voltage of strong strings as the target voltage. The selectoperation (3702) can be carried out by LSMUs or the management unit. Inan embodiment, strong strings and weak strings (having string outputvoltage not within the range from the output voltage of the strongeststring) can be identified. Having identified one or more strong strings,the string output voltages from weak strings can be upconverted in orderto cause the solar array output voltage (3415) of the parallel bus(3450) to equal (or match or converges on) the string output voltage(3414, 3424) of the strongest string. Identification of strong stringsis not a required operation since such identification can have occurredbefore the method (3700) begins.

The operate operation (3704) can include operating a first LSMU in abypass mode. The first LSMU converts the string output voltage for astrongest string or a strong string (a string having a string outputvoltage that is within a range from the output voltage of the strongeststring). In an embodiment, the first LSMU is an LSMU that can bebypassed to increase the total power output of the strings. Bypass modecan be used when the solar array target voltage equals or matches thestring output voltage of the strongest string or when the solar arraytarget voltage is within the range from the output voltage of astrongest string. The bypass mode can be used to allow current to passthrough or around a first LSMU without a voltage drop due to currentpassing through high resistance or high impedance circuitry in the firstLSMU. When an LSMU operates in bypass mode, the string operates at thevoltage of the parallel bus, which may be different from the maximumpower point voltage of the string. In other words, there can be a smallpower loss.

The method (3700) can include an instruct operation (3706) where asecond LSMU is instructed to upconvert a second string output voltage sothat the solar array output voltage equals the target voltage. Thesecond LSMU can be connected to a second string. The second string canbe a weak string or a string having a string output voltage that is notwithin a range from the output voltage of a strongest string. Theinstruct operation (3706) can include providing instructions (signals orcommands) to the second LSMU to upconvert the second string outputvoltage. In an embodiment, a management unit provides the instructionsto the second LSMU. In an embodiment, a controlling LSMU provides theinstructions to the second LSMU. In an embodiment, the controlling LSMUis the second LSMU, so the second LSMU provides the instructions toitself.

After the instruct operation (3706), the method (3700) can repeat orloop. Alternatively, the method (3700) can wait for one or more loops ofan MPPT algorithm to run. Once the MPPT algorithm has run one or moretimes, solar modules on the string can be balanced, and the method(3700) can begin again. The inverter carries out the MPPT algorithm. Inan embodiment, the MPPT algorithm runs at the same time that the method(3700) operates. In other words, the inverter tries to find the solararray's MPP while the solar array is balancing currents produces bysolar modules on each string and balancing voltages between theparallel-connected strings. The MPPT algorithm does not have to stop forbalancing of solar modules and strings to take place.

In one embodiment, an inverter is allowed to be powered by two (or more)separately controlled strings, connected to the parallel bus (3450) viarespective LSMUs, and to operate with its internal MPPT algorithm (thatis typically designed for a single string) to deliver optimal or nearoptimal overall system performance. This arrangement does not requirethat the inverter's internal MPPT be turned off or the need to move theinverter to a fixed input voltage, and also not require a separate MPPTper string. In one embodiment, the inverter is to operate at its optimalspot continuously, as power generation conditions change constantly; andhence the system performs continuous rebalancing to achieve optimaloperation.

In one embodiment, the inverter's MPPT algorithms pull and tug currentto create current changes delta current (dl). The algorithm to controlthe LSMUs has several operations to calculate the dl for every stringand to cause the corresponding current change by adjusting PWM value onLSMUs connected to weaker string(s). The inverter takes furtheroperations to get to the maximum power point.

For example, in one embodiment, the delta current of each string (dIk)is calculated when the string is connected via an LSMU. Weak strings areidentified. It may be checked whether disconnecting those strings wouldproduce more power. The average delta current is then calculated for theweak strings (dIW). For each strong string, adjust or keep PWM values ofits LSMU to be 100% or near 100% (minimal or no change the string).Other strings are adjusted to match voltage set by strong string bychanging their voltage output driving PWM to a different value than100%. Wait some time to allow the inverter to run several MTTP cycles,before repeating the operations to balancing the strings.

In one embodiment, the decision of whether to up-convert in some casesthe strong string may depend on several factors, including weak solarirradiation, temperature, and the parameters of the string vs. theinverter, amongst other consideration, but the decision is not limitedto these factors.

In an embodiment, an MPP for the strong string can be determined. Thestrong string can then operate at its MPP. In an embodiment an invertercan run an MPPT algorithm to determine the MPP for the strong string andthe strong string can then operate at its MPP.

In an embodiment, the method (3700) can include identifying one or morestrong strings and one or more weak strings. Strong strings and weakstrings can be identified as previously described (see discussion ofFIG. 26 ). In an embodiment, the weak string can be disconnected fromthe solar array, and a resulting change in the power output of the solararray monitored to see if disconnecting the weak string improves thesolar array power output. Disconnecting can include setting a duty cycleof a switchable connection (e.g., transistor) in the LSMU to zero(always off or open). It should also be understood that while the method(3700) refers to only a single strong string and a single weak string,the method can also be applied where there is more than one strongstring and/or more than one weak string.

To further improve the energy production performance of photovoltaicsystems and/or reduce the cost of the systems, an enhanced system andmethod is provided for measuring relevant data of any string to helpmanagers decide whether to add or remove an LSMU from a string. FIG. 38illustrates an overview of an exemplary system (3800), according to oneaspect of the system and method described herein. Strings (3410 a-n) arecomplete strings with LMUs, etc., as illustrated in FIG. 34 anddiscussed throughout. As described in more detail below, each of thestrings (3410 a-n) is connected or routed to one of the removablemodules (3804 a-n) through connection pairs (3803 a-n) corresponding tothe power input cables from the respective strings (3410 a-n) of solarpanels. In one embodiment, the modules (3804 a-n) are installed in thecombiner box (3801) and configured to be in the form of blades, or thinmodular electronic circuit boards, and are enclosed in a bladeenclosure, such as cage (3802), capable of holding multiple blades andconfigured such that an operator can insert a blade into or remove ablade out of the cage (3802).

The modules (3804 a-n) implemented in the form of blades can be simplebypass blades which can simply facilitate an electric connection betweenthe strings (3410 a-n) and the load bus (3805), bypass blades withmeasurement and disconnect features, or bypass blades with measurementand disconnect features as well as an LSMU, such as the bladeillustrated in FIG. 34 . In one embodiment, an operator can easilyremove a blade from the cage (3802) by pulling the blade away from thecage (3802), or by other non-destructive ways to disengage/uninstall theblade from the cage (3802), such as pushing one or more levelersconfigured to eject the blade from a slot connector or receptacle inwhich the module is installed/inserted.

In an embodiment, modules (3804 a-n) implemented in the form of bladeshave five connections/connectors through which the blades are connectedto other system components in the cage (3802), which provides a housingto the components disposed therein. The connections include twoconnections (3803 a-n) (one pair each) through which each module (3804a-n) implemented in the form of a blade is connected to thecorresponding string (3410 a-n); two connections through which eachmodule (3804 a-n) implemented in the form of a blade is connected to theload bus (3805); and one connection through which each module (3804 a-n)implemented in the form of a blade is connected to local controller(3810) through a controller bus (3808). The load bus (3805) is connectedto the inverter or other energy consuming unit (3806).

As illustrated in FIG. 38 , the local controller (3810) communicateswith a central controller (3807) of the overall system. Thiscommunication may be achieved by a wired connection or a wirelessconnection. Even though controller bus (3808) is illustrated as a singleunit in FIG. 38 , in other embodiments, controller bus (3808) maycomprise of discrete lines (not shown). In another embodiment (notshown), the system (3800), rather than having a single local controller(3810), has a small controller in each module (3804 a-n) implemented inthe form of a blade. In other embodiments, each module (3804 a-n)implemented in the form of a blade may have its own local controller(not shown), and the local controller may employ a simple communicationprotocol such as, for example, an 12 bus or similar to communicate withlocal controller (3810). In one embodiment, some of the functions of theLSMU as discussed above (e.g., described in connection with FIG. 34 )are moved from the modules (3804 a-n) implemented in the form of bladesto the local controller (3810) to reduce the cost of the modules (3804a-n) implemented in the form of blades and the cost of the overallsystem (since the modules (3804 a-n) implemented in the form of bladescan share the functions implemented in the local controller (3810)through the controller bus (3808)). In one embodiment, some functions ofthe LSMU as discussed above are implemented in one (or more) of themodules (3804 a-n) implemented in the form of blades; and other modules(3804 a-n) implemented in the form of blades without such functions areconfigured to communicate with modules (3804 a-n) implemented in theform of blades having such functions in a way to communicate with thelocal controller (3810) to implement such function. Such a locationcontroller (3810) can be implemented in one or more of the removablemodules (3804 a-n) implemented in the form of blades in one embodiment.

FIGS. 39A and 39B illustrate measure-and-disconnect andmeasure-and-disconnect-with-LSMU module types, respectively, accordingto various aspects of the system and method disclosed herein. FIG. 39Aillustrates the measurement-and-disconnect module having avoltage-measuring unit (3914) for measuring voltage, a current-measuringunit (3913) for measuring current provided by a string connected to thatmodule, and a switch (3912) for connecting or disconnecting the stringto or from the load bus (3805). The measuring units (3914 and 3913) andswitch (3912) can interact with a local processor (3911) which connects,via the controller bus (3808) back to the local controller (3810), whichin turn connects back to the central controller (3807), as illustratedin FIG. 38 .

FIG. 39B illustrates an embodiment similar to that of FIG. 39A, exceptthat it includes an upconversion unit (3926) (e.g., as in an LSMUdiscussed above in connection with FIG. 34 ), and a bypass switch (3925)that can be activated by controller (3921) to bypass upconversion unit(3926). Switch (3922) is analogous to switch (3912) of FIG. 39A, and isutilized for connecting or disconnecting the string to the module. Inone embodiment, the switches (3925, 3922, and 3912) are mechanicalswitches; in other embodiments the switches (3925, 3922, and 3912) mayinclude solid-state switches instead of mechanical switches, or othertypes of switching devices. Thus, the switches illustrated in thedrawings are not necessarily mechanical switches. In some embodiments, abypass diode (not shown) may be utilized.

FIG. 40 illustrates an overview of a case (4000) which can be utilizedto encase the cage (3802) of FIG. 38 , according to one aspect of thesystem and method described herein. Cage (3802) can be mounted in thecenter of case (4000). The illustrated embodiment of FIG. 40 furtherincludes screw terminal block (4001), to which the load bus (3805) isconnected, as well as screw terminal blocks (4002 a-n), to which thestring cables (3803 a-n) are connected. In some embodiments, the loadbus (3805) and the string cables (3803 a-n) are secured to screwterminal blocks (4002 a-n) with screws, and in other embodiments, theconnection can be made with special secure fasteners and/or connectors.The screw terminal blocks (4002 a-n) supply the current into thebackplane board (4003) of cage (3802). As illustrated in FIG. 40 , thebackplane board (4003) can be a printed circuit board (PCB) secured inthe cage (3802) into which the modules (3804 a-n) implemented in theform of blades are connected by sliding into connectors (4004 a-n). In apreferred embodiment, an operator can easily remove a blade from thecage (3802) by pulling the blade away from the connectors (4004 a-n). Inone embodiment, connectors (4004 a-n) have multiple contacts for each ofthe power connectors. Connectors (4004 a-n) can also have additionalspacing between those connections due to the high voltage and requiredcreepage distance, and can have as many connections as needed to thecontroller bus (3808). The controller bus (3808) is connected to thelocal controller (3810). In some embodiments, the local controller(3810) can be a separate module also plugged into backplane board(4003), while in other embodiments, the local controller (3810) can beimplemented directly on the backplane board (4003). Further, in someembodiments, the screw terminals (4002 a-n) can be secured directly onthe modules (3804 a-n) implemented in the form of blades instead of onthe backplane board (4003), thereby reducing the number of necessaryinterconnections.

FIG. 41 illustrates an overview of an exemplary process flow (4100),according to the system and method disclosed herein. In step (4101), allmeasurement-only blades are installed. In step (4102), the systemoperates for a certain period by trimming the LMUs, thereby essentiallyreducing the voltage on the strong strings to match the voltage on theweaker strings. The time period during which the system operates withall measurement-only blades can be as short as one or two days, a fewmonths, or as long as several years, or otherwise before a string startsto degrade due to aging. After the period of operation (4102) haselapsed, the string performance data is analyzed in operation (4103). Inlight of the analysis performed in operation (4103), suggestions aremade identifying weak strings to receive a blade with an LSMU forvoltage upconversion in operation (4104). In one embodiment, thesuggestions are made based on comparing the measurements to apredetermined target direct current range, which can be a target voltagerange, a target power range, or a target current range for the parallelbus, as discussed throughout herein. In operation (4105), after theinstallation of blades on weak strings, the analysis of stringperformance data resumes and continues. The process then cycles back tooperation (4102) after a certain period of time, such as, for example,every two or three months, and the system operation is again analyzedand modified as described above. In some embodiments, the review andanalysis cycle described above may be used to adjust the strings in thesystem as needed for seasonal changes in solar power and position. Inother embodiments, the review and analysis cycle is utilized inidentifying and mitigating the adverse effects of panel aging in certainstrings.

FIG. 42 illustrates an overview of exemplary feature set (4200) ofmodule (3804 x) implemented in the form of a blade, according to oneaspect of the system and method disclosed herein. As explained above,the blade utilized in some embodiments described herein can be a simplebypass blade, a bypass blade with measurement and disconnect features,or a bypass blade with measurement and disconnect features as well as anLSMU, such as the blade illustrated in FIG. 34 . As such, module (3804x) implemented in the form of a blade may or may not have all componentsof an LSMU illustrated in FIG. 34 .

In one embodiment, a switch device (4201) enables a user to push abutton, for example, to trigger a system switch such as switch (3912) or(3922) illustrated in FIGS. 39A and 39B, respectively, to turn off thecurrent before removing a blade for exchange or maintenance, thusavoiding any arcing. In one embodiment, an indicator light (4202) (orother suitable indicator, for example electrophoretic, LCD, etc.) can beutilized to indicate the on/off status of the unit. Connector (4004 x),as illustrated herein in greater detail, includes two sections with thecontroller section (4203) illustrated as a separate piece. A person ofordinary skill in the art would appreciate that such separation is notnecessary, but in many cases, additional spacing is desired for galvanicseparation for creeping current, etc. In one embodiment, fourhigh-powered connectors (4204 a-n) fit into a section whose widthaccommodates all of them, with the width necessary to provide multiplecontacts for each of the higher currents and voltages in a typicalsystem. In some embodiments, spaces between each connector may be addedto provide better galvanic separation.

In one embodiment, two short pins (4205 a-n) are grouped with thehigh-powered connectors (4204 a-n). These short pins (4205 a-n) can be,for example, half-length, so as to make contact when module (3804 x)implemented in the form of a blade is fully inserted. If module (3804 x)in the form of a blade is removed without prior deactivation of thecircuit, disconnection of short pins (4205 a-n) would occur prior to thedisengagement of the high-powered connectors (4204 a-n) and thusimmediately cause the controller (e.g., 3810, 3911, or 3921) to breakthe circuit before the main power is disrupted, thus giving thecontroller (e.g., 3810, 3911, or 3921) time to turn off switches (3922)and (3912).

FIG. 43 illustrates an exemplary alternative embodiment (4300) of cage(3802), previously described in relation to FIG. 40 , according to oneaspect of the system and method disclosed herein. Rather than installingone long cage that often is only partially used, in a modular approach,multiple cages may be installed, with each cage unit having a pluralityof receptacles or connector slots to receive the removable modules. Thereceptacles or connector slots have connectors corresponding to theconnectors (e.g., 4203, 4205, 4204) of a removable module. When theremovable module is installed in the receptacle or connector slot, theconnectors of the removable module and the connectors of the receptacleor connector slot make contact with each other to form electricalconnections between the circuitry on the removable module and thecircuitry on the cage (3802). In one embodiment, the case (4000)includes a controller cage unit (4301) having a controller (3810), andtwo slave cage units (4302 a) and (4302 b) each having six receptacleslots. One skilled in the art would appreciate that the number of slotsis purely exemplary. In fact, a box may contain, for example, threecages with 12 slots each, for a total of 36 strings, etc. However, inone embodiment, the slave cage units contain six slots, because currentcombiner boxes are typically sized in increments of six. While asillustrated in FIG. 43 , slave cage units (4302 a) and (4302 b) do notcontain their own controller (3810) and rely on internal connections(4310 a-n) in a daisy-chain format to connect to controller cage unit(4301), in other embodiments, more than one slave cage unit may containa controller. As illustrated in FIG. 43 , the power bus (4303) isconnected by daisy-chaining the plug-in or screw-on cables to the cageunits (4301, 4302 a and 4302 b), and each cage unit (4301, 4302 a and4302 b) has its own connections (4304 a-n) for the string cables (3803a-n).

FIG. 44 shows an exemplary process (4400) for operation of the removablemodule unit, according to one aspect of the system and method disclosedherein. The system is initialized (4401) after the strings (3410 a-n)are connected to the modules (3804 a-n) implemented in the form ofblades. The blade unit is turned on (4402), which can be done by switch(3912) or (3922). Measurements are received (4403) in the controller(e.g., 3911 or 3921) of the blade from amp and voltage meters (if bothare present; in some cases, only one or the other type of meter may bepresent) of the blade. The measurements are sent to the local controller(3810) from the controller (e.g., 3911 or 3921) of the blade via thecontroller bus (3808). The system (e.g., the controller (3911) or (3921)of the blade, the local controller (3810), and/or the central controller(3807)) checks (4405) to determine if the switch (3912) or (3922) or theshort pins (4205 a-n) on the blades have been activated (e.g., in aposition indicating that the blade is not fully seated in the receptacleor connection slot). If it is determined (4406) that either isactivated, the blade is disconnected by turning off switch (3912) or(3922). The system (e.g., the controller (3911) or (3921) of the blade,the local controller (3810), and/or the central controller (3807))continuously checks (4405) the switch (3912) or (3922) or the short pins(4205 a-n) until the blade is fully seated. If the switch (3912) or(3922) or the short pins (4205 a-n) on the blades have not beenactivated (e.g., in a position indicating that the blade is fully seatedin the receptacle or connection slot), the blade is kept on or turned on(4407) by turning on the switch (3912) or (3922). When the switch (3912)or (3922) is turned on or off (to connect or disconnect the string fromthe load bus (3805)), the system (e.g., under the control of thecontroller (3911) or (3921) of the blade, the local controller (3810),and/or the central controller (3807)) may turn light (4202) on or off,according to the status of the switch (3912) or (3922). Further, in somecases, arc detection circuitry may be added to either the LSMU or thebypass module. Using arc detection/prevention is functionally orthogonalto some of the LSMU/bypass card functions discussed above, butcomplements the functionality nicely. In some cases arc detection maycover the whole box only, in lieu of each string separately.

FIG. 45 illustrates an exemplary overview of a photovoltaic system(4500) having a plurality of strings (e.g., 4504 and 4514) for supplyingelectric power to a load (4510), which among other things, can be apower grid, a string combiner, or an inverter. The system utilizescombined local management units (CLMUs) (e.g, 4502 a-n), each configuredto operate more than one solar panel (e.g., 4501 a-c) to the string bus(4503), as compared, for example, to the system illustrated in FIG. 34 ,wherein each solar panel is connected to and managed by a separate LMUdedicated thereto. For purposes of clarity and simplicity, the systemillustrated in FIG. 45 includes two sets of series connected CLMUs (4502a-n and 4512 a-n), with each CLMU connected between a three-panel group(e.g., 4502 a-c) and the string bus (4503). One skilled in the art wouldrecognize that the system disclosed herein is not limited to theillustrated exemplary system, and may include additional or fewerstrings and CLMUs, or groups of solar panels with more or fewer solarpanels connected to the CLMUs.

In one embodiment, the strings (e.g., 4504 and 4514) can be received inand managed via the LSMUs (e.g., 4505 or 4515), which can be any of theLSMUs disclosed herein. For example, a modular unit string managementsystem as illustrated in FIGS. 38 and 40 can be used to implement theLSMUs (e.g., 4505 or 4515) in a combiner box.

In one embodiment, the system management unit (4520) can be utilized todetermine one or more operating parameters (e.g., duty cycles, phases,synchronize pulses, etc.) based on measurements of the operatingvoltages and currents of the solar panels as reported by the individualCLMUs (e.g., 4502 a-n). A CLMU (4502 a) is configured to determine itsown duty cycles for operating the respective solar panels (e.g., 4501a-c) with or without relying upon communicating with other managementunits. The duty cycles are configured to separately operate the solarmodules in the group connected to the CLMU at the maximum power point.

FIG. 46 illustrates an exemplary overview of a system (4600) whichutilizes a CLMU housed inside a junction box (e.g., 4614) of a solarpanel (e.g., 4604) to manage the solar panels (e.g., 4603, 4604 and4605) that are connected to the CLMU. For purposes of clarity andsimplicity, the exemplary system illustrated in FIG. 46 includes athree-panel group consisting of Panel A (4603), Panel B (4604), andPanel C (4605). One skilled in the art would understand that the systemdisclosed herein is not limited to a single group of panels, or tothree-panel groups. For example, the system disclosed herein may includea group of panels in a string having more or fewer panels than three.Also, the disclosed system can include more than a single group ofpanels with each group connected to a CLMU.

In one embodiment, the CLMU functions as an LMU discussed above for eachindividual solar panel (e.g., 4603, 4064 or 4605) to separately operatethe solar panels/solar modules. However, combining the LMU functions ina CLMU allows the sharing of common components and reducing of thenumber and cost of management units present in the system.

In the exemplary system (4600) illustrated in FIG. 46 , the junction box(4614) on Panel B 4604 houses the CLMU. In one embodiment, the CLMU ishoused in the junction box of the panel physically located in the middleor center of the group of panels connected thereto (to reduce the wirelength required to connect the panels (4603, 4604 and 4605)). If thereare an even number of panels in the group, the CLMU can be housed in thejunction box of one of the middle panels.

In one embodiment, the CLMU is secured in a location separate from theindividual solar panels connected thereto (as illustrated in FIG. 45 ).For example, the CLMU can be installed at a location at or near thecenter of the region defined by the locations of the junction boxes ofthe solar panels controlled by the CLMU. Alternatively, the CLMU can beinstalled inside the junction box of the solar panel that is locatedclosest to the center of the regions defined by the locations of thejunction boxes of the solar panels controlled by the CLMU.

In FIG. 46 , the positive and negative output terminals of the junctionbox (4614) of Panel B (4604) are connected to the wires (4601, 4602) ofthe string bus leading to other solar panels in the string. The positiveand negative output terminals of Panel A's junction box (4613) areconnected to a pair of input terminals of Panel B's junction box (4614)through a pair of wires (4633). Similarly, the positive and negativeoutput terminals of Panel C's junction box (4615) are connected toanother pair of input terminals of Panel B's junction box (4614) by apair of wires (4635). Local wires (4623, 4624, 4625) are used to connectthe solar modules of the respective Panels A, B and C to a further pairof input terminals of their respective junction boxes (4613, 4614,4615). In Panel B, the CLMU installed in the junction box (4614)connects the input terminals to the output terminals. In Panels A and C,the junction box (4614) of Panel B connects their input terminals(corresponding to local wires 4623 and 2625) directly to their outputterminals. Thus, the solar modules of Panels A, B and C are operated andcontrolled by the CLMU located inside the junction box (4614) of PanelB. Thus, Panels A and C are connected to the string via the junction box(4614) of Panel B that houses the CLMU.

FIG. 47 illustrates an exemplary junction box (4700) encasing the CLMU,similar to the junction box (4614) of Panel B in FIG. 46 , according toone aspect of the system and method disclosed herein. Similar to thediscussion of FIG. 46 , and for purposes of clarity and simplicity, theexemplary system illustrated in FIG. 47 also includes a three-panelgroup. In FIG. 47 , three direct current converters (4705, 4706, 4707)are connected to the solar modules (4603, 4604, 4605) of three separatepanels (e.g., Panels A, B and C illustrated in FIG. 46 ), respectively,through three connector pairs (4708 a-c) (e.g., input terminalscorresponding to the local wires (4623, 4624, 4625) illustrated in FIG.46 ). One skilled in the art would understand that the system disclosedherein is not limited to a single group of panels, three-panel groups,or the corresponding number of converters connected thereto. More orless converters can be included in the CLMU for controlling/operatingmore or less panels as a group.

In one embodiment, the junction box (4614) is secured to the panelphysically located in the middle of the panel group layout. Becausedifferent configurations are possible, for example, with two, four,five, or any other suitable number of panels, the illustratedpositioning may be different in other embodiments of the disclosedsystem.

In FIG. 47 , the outputs of the converters (4705-4707) are connected inseries; and the connectors (4709 a and 4709 b) connect the seriesconnected converters (4705-4707) to the wires (4601 and 4602) leading toother panels in the string.

In FIG. 47 , the CLMU housed in the junction box (4614) includes controlcircuitry which includes a controller (4701) and a communications unit(4702). The controller (4701) operates the converters connected thereto(e.g., 4705, 4706, and 4707) according to their separate duty cycles. Inone embodiment, controller (4701) is configured to control allconverters connected thereto (e.g., 4705, 4706, and 4707). In oneembodiment, configuring the controller to implement themultiple-converter control function is accomplished by modifying thecontroller's software. In one embodiment, the multiple-converter controlfunctionality is implemented by utilizing a controller capable ofrunning multiple independent software instances, each instancecontrolling one converter.

The converters (e.g., 4705, 4706, and 4707) may be any of the typesdescribed herein throughout. For example, the converter (e.g., 4705,4706, or 4707) can be an LMU (101) illustrated in FIGS. 1-3B without thecontroller (109); and the controllers (109) of the respective LMUs (101)can be combined via the controller (4701) and the communications unit(4702) to reduce cost. Thus, through sharing the common components(e.g., the controller and the communications unit), the cost of thesystem is reduced.

Also, the methods disclosed above to individually control or operate thesolar modules of separate panels can be used accordingly with a CLMUhosted inside the junction box (4614).

Communications unit (4702) may use a wireless connection (4703), or awired connection (4704) to the string (4601), to communicate with asystem management unit (e.g., (204) illustrated in FIG. 4A, FIG. 4B, andFIG. 11A, (3440) illustrated in FIG. 34 ). In one embodiment,communications unit (4702) is configured to enable the single controller(4701) to control more than one converter (e.g., 4705, 4706, and 4707)by facilitating the controller's communications with the systemmanagement unit. In FIG. 47 , the converters (4705, 4706, and 4707)share a controller (4701). Alternatively, each converter may have itsown controller.

In one embodiment, galvanic separation may be used on the wires betweenthe controller and the converters for creeping current etc.

While the input voltages of solar modules (4603-4605) connected to theconverters (4705-4707) of the CLMU may be different, the output currentsof the converters (4705-4707) are the same, due to the series connectionof the outputs of the converters (4705-4707). Thus, a single currentmeasurement unit/current sensor can be used to measure the outputcurrent of the solar panels connected to a CLMU.

In one embodiment, the CLMU is configured to use a voltage measurementunit for multiple solar panels connected to the CLMU via a set ofswitches. The CLMU controls the states of the switches to dynamicallyconnect the voltage measurement unit to one of the converters(4705-4707) at a time to measure the input or output voltage measurementof the respective solar panel.

In one embodiment, the system may have a combiner box to combinemultiple strings of panels that are connected via CLMUs, in a way asillustrated in FIGS. 38-44 .

In FIG. 47 , the CLMU controls/operates the solar modules (4603-4605) ina way similar to a cluster of LMUs, each separatelycontrolling/operating a respective one of the solar modules (4603-4605).Integrating the cluster of LMUs into the CLMU housing in a singlejunction box allows the cluster of LMUs to share hardware components(e.g., the communications unit (4702) and at least part of thecontroller (4701)) and thus reduces the cost of the system.Alternatively, the CLMU may have a number of complete, separate LMUs inone housing.

FIG. 48 shows an exemplary simplified system 4800 for string balancingaccording to a further aspect of this disclosure. The bus voltage V_(B)4803 is set to the voltage V_(Smax) corresponding to the strongeststring. In this example the strongest string, Sn, is string 4801 n. Theweaker strings S1 through S2 and so forth in between each have an LMU4type of up-converter module 4802 a-n. A distinction of the LMU4 typeconverter from other up-converting systems is that LMU4 modules 4802 a-ndo not up-convert voltage to a fixed voltage, but rather, to whateverthe voltage is on the strongest string. These LMU4 modules mayalternatively measure the string output and compare to the bus voltageto determine the appropriate voltage. They may also be configured toincorporate a communication mechanism to the other LMUs so as to arriveat the strongest string voltage around and just above the bus voltage.In some cases, all strings may have modules and the modules may simplyhave a by-pass module, which causes less power loss than theup-conversion module. Typical conversion losses may be in the range of 1to 3 percent with enhanced designs; with standard designs, the lossesmay be in the range of 2 to 5 percent. By-pass loss should be far lessthan one percent, so it is worth the expense of having a dedicatedmodule for a by-pass switch option. The discussion above, with referenceto FIGS. 37-47 , explains how these elements may be designed modularlyas blades or other plug-in type modules, but all such design variations,whether in modular form or just as boxes inside a combiner box, are thesame for the purposes of the string balancing system and methoddisclosed herein.

FIGS. 49A and 49B show a bus architecture 4900 for a string withup-converter, according to one of these existing approaches. FIG. 49Ashows the up-converter 4802 x. It takes the whole power of the moduleS_(X), made of V_(SX) and I_(SX), and up-converts to V_(B), which, asdiscussed above, is V_(Smax). Also shown in FIG. 49A are the elements4901 a-n that represent the whole string.

FIG. 49B shows a quick calculation of the losses in this example. At abus voltage of 550Vdc (typical maximum string voltage for a 600-voltnominal bus system), and a V_(SX) of this particular module 4901 x of480 volts and an I_(SX) of 8A, the power coming out of the string P_(SX)is 8×480 watts=3840 watts. At a loss of about 3 percent, for anaggressive design, the loss may be figured as 3 percent of 3840 watts,equaling 115.2 watts, which is quite a substantial loss. Also, thehigher loss further increases the heat in the combiner box, thus furtherreducing the lifetime of the components, contributing to earlierfailure, loss of power production, and higher operating costs.

FIGS. 50A and 50B show an up-converter architecture 5000, according toan embodiment of the system and method disclosed herein. In FIG. 50A,instead of converting up the string as a whole, an additional module5001 simply supplements the missing voltage through a dc-to-dcconverter, producing in the same current I_(SX) and creating a V_(out)that brings up the module voltage V_(SX) to the bus voltage V_(B) (whichis at V_(Smax)). Module 5001 x is a dc-to-dc converter. It has a loss ofP_(LX) that is calculated in the discussion following. It has a supplyvoltage V_(supply) and has to bring in the missing voltage. In somecases V_(supply) may be connected directly to the module P_(SX), meaningthat the current I_(SX) is reduced and actually further improvesefficiency. In other cases, V_(supply) comes from the bus V_(B), whichapproach is a little less efficient than the previous approachdescribed, but more secure for stable operation. In yet other cases, itmay come from some other supply source, or even from the grid.

FIG. 50B shows the power configuration. As described in the discussionof FIG. 49B, the bus voltage V_(B), again, which is at V_(Smax), is 550volts and the module voltage V_(SX) is 480 volts. The V_(out) of module5001 x is approximately 70 volts. At the current I_(SX) of 8 amps, thepower P_(CX) that must be supplemented is 560 watts. These figuresassume that no power is used from the module 5001 x, because if powerwere used from the module, I_(SX) would be reduced by the amount thatmust be taken out to produce the 560 watts, which amount would be about15 percent. Even not reducing the total power by this number, the powerproduced by the converter P_(CX) (power converter x) is 560 watts. At a3 percent loss, power loss P_(LX) is 16.8 watts, which is approximately15 percent of the loss calculated in FIG. 49B. Connecting V_(supply) toV_(SX) directly reduces the current that has to go through the module bythe current pulled into the module by production, and therefore theefficiency further rises.

FIGS. 51A and 51B show a further analysis of the system described in thediscussion of FIGS. 50A and 50B, with, in this case, the supply derivedfrom the string itself. In FIG. 51A, if the V_(supply) is connected by aconnection 5101 to the module directly, the current supplied by themodule I_(SX) is split into I_(SUPP), which is the supply current forthe module, and I_(BX), which is its contribution to the bus current.Since the power is in the 10- to 15-percent range, that means that thepower converted has to be reduced.

FIG. 51B shows that after all the considerations, the supply current isslightly greater than 1 amp, resulting in an I_(BX) of about 7 amps.Thus the effective power that must be added is 7 amps times 70 volts,which is 490 watts. At a 3 percent loss, this approach renders aconversion loss of only 14.7 watts. That result is an additionalimprovement of 10 to 15 percent, because the power used by theup-conversion module 5001 x has now been taken out of the power to beconverted, and therefore the losses are further reduced.

In an alternative embodiment, the units 5001 x could be supplied from asingle converter (as a low voltage bus for all top-off converters 5001 xin parallel), rather than as is shown in FIGS. 51A and 51B which is doneon a string by string basis. The tradeoffs are that the low voltage busshown in FIGS. 50A and 50B is less efficient, but advantageously lesscostly, as the components for HV-LV conversion are more expensive. Theconfiguration in FIGS. 50A and 50B, utilizing a low voltage (LV) bus(not shown) would result in cheaper units 5001 x, but a bit lessefficient. The embodiment in FIGS. 51A and 51B is more efficient, butalso substantially more expensive, since now all the units 5001 x in theembodiment shown in FIGS. 51A and 51B have to be manufactured as highvoltage (HV) components. So cost of components and cost of energy willdetermine which one is better over the lifetime of the system. As bothfluctuate, then either one may be more advantageous in a specificsituation.

It is clear that many modifications and variations of this embodimentcan be made by one skilled in the art without departing from the spiritof the novel art of this disclosure. For example, the systems and methodherein disclosed can be applied to energy generating systems besidesphotovoltaic systems (e.g., windmills, water turbines, hydrogen fuelcells, to name a few). Also, although the target voltage has beencompared to the average string output voltage of strong strings, otherreference points for the target voltage can also be used. Somenon-limiting examples include an string output voltage for the strongeststring or a median string output voltage of strong strings.

In the foregoing specification, the disclosure has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications can be made thereto without departing fromthe broader spirit and scope as set forth in the following claims. Thespecification and drawings are, accordingly, to be regarded in anillustrative sense rather than a restrictive sense.

What is claimed is:
 1. A photovoltaic system, comprising: a plurality ofstrings, each string comprising a plurality of photovoltaic panels thatprovide a string output for a respective string; respective converterscoupled between corresponding string outputs and a bus, each respectiveconverter configured to convert a string output voltage at the stringoutput to a bus voltage at a converter output of the respectiveconverter; and each converter comprises a bypass switch configured toconnect the string output to the bus when the string output is equal tothe bus voltage.
 2. The photovoltaic system of claim 1, wherein the busis powered by the plurality of strings in parallel.
 3. The photovoltaicsystem of claim 1, wherein each converter is part of a respectivelogical management unit (LMU), wherein each LMU is configured to measurethe string output voltage and compare the string output voltage to thebus voltage.
 4. The photovoltaic system of claim 1, wherein eachconverter is part of a respective logical management unit (LMU), whereineach LMU has a communication unit to receive data indicating the stringoutput voltage.
 5. The photovoltaic system of claim 4, wherein eachcommunication unit is wireless.
 6. A method of balancing string outputs,the method comprising: coupling a converter to a string output of afirst string, the first string comprising a plurality of photovoltaicpanels that provide the string output to the converter; coupling theconverter to a bus; converting, by the converter, a string outputvoltage at the string output to a bus voltage at a converter output ofthe converter; and connecting the string output directly to the bus bybypassing the converter with a switch when the string output is equal tothe bus voltage.
 7. The method of claim 6, wherein the bus is powered bya plurality of parallel strings including the first string.
 8. Themethod of claim 6, further comprising measuring the string outputvoltage of the first string and comparing the string output voltage ofthe first string to the bus voltage.
 9. The method of claim 6, whereinthe converter is a part of a logical management unit (LMU), wherein theLMU has a communication unit to receive data indicating the bus voltage.